TILLEY, CECIL EDGAR

(b. Adelaide, South Australia, 14 May 1894; d. Cambridge, England, 24 January 1973)

mineralogy, petrology.

Tilley was the son of a civil engineer in the South Australian State Service and so spent his early years in various parts of South Australia and at Darwin in the Northern Territory of Australia, where his father was concerned with harbor construction. The family was very musical, and as a boy he was a proficient pianist and organist. When Tilley entered the University of Adelaide in 1911, his enthusiasm for science had already developed and displaced, much to his subsequent regret, his musical intrests. The events that dierected his scientific interest toward geology and petrology are unrecorded. The geology department at Adelaide was small but distinguished; Walter Howchin was a stratigrapher of some eminence, and Douglas Mawson, a petrologist, was away leading the Australian Antarctic Expedition (1909–1913), which quickly earned him a knighthood. In his second year, 1912, Tilley accepted the post of cadet in the geology department; his fees were remitted in exchange for his performing several minor duties, including making thin sections. During Mawson’s absence his locum tenens was W. R. Browne, who quickly recognized Tilley’s ability.

Having completed the B.Sc. Course in November 1914, he transferred to Sydney University, to which Browne had returned in 1913, and took the final year B.Sc. courses in chemistry and geology; he was awarded medals in both subjects and was appointed a junior emonstrator in geology and mineralogy. He held the post for only one year and then, at the end of 1916, made his first voyage to Britain to work as a chemist for the Department of Explosives Supply at Queensferry, near Chester, where, in charge of a chemical plant at the early age of twenty-three, he had his first taste of being in command. After the Armistice he returned to Sydney to resume his demonstratorship in geology and mineralogy. After a year he was on his way back to Britain with a 1851 Exhibition Scholarship to do petrological research under Alfred Harker in Cambridge.

Physically Tilley was a large man, tall and broadshouldered. In youth he was shy and did not make friends easily. In middle life his manner was reserved in his own department, but outside it he relaxed and at parties his great voice could be heard booming above the general hum of conversation. After his marriage in 1928 to Irene Doris Marshall, he kept his private and professional lives separate. He was a man of regular habits, arriving early at his laboratory and staying late into the night with breaks only for afternoon tea at home with his wife and daughter Anne and for dinner in Emmanuel College. He was dedicated to his work from an early age and had no hobbies. He appeared to have a photographic memory, never forgetting anything he had read or any mineral he had once seen. He traveled widely, exclusively in connection with his work.

After acting as Browne’s field assistant in early 1913, Tilley went to investigate the granites of Cape Willoughby on Kangaroo Island. The result, his first published petrological work, appeared in two papers in 1919. After Browne’s return to Sydney in 1913, pupil and teacher began a correspondence that lasted for over a decade; fragments survive in the possession of T. G. Vallance. Browne exercised a strong formative influence on the first decade of his pupil’s career.

On 5 February 1920 Tilley first set foot in Cambridge. He was taken into the “argust presence of Harker,” who, as he wrote to Browne, “had none of the severity with which I had associated the name.” At this time Harker was reader in petrology in the Sedgwick Museum of Geology and had lately transferred the attention of his original and rigorous mind from igneous to metamorphic petrology. He was to provide the second major influence on the development of Tilley’s career. Tilley had brought to Cambridge an extensive collection of metamorphic rocks from Southern Eyre Peninsula, South Australia, on which he based five papers (1920–1921), which brought him to the notice of the geological world; these were followed by a wealth of publications over the next thirty years that established his leadership in metamorphic petrology.

Tilley’s dissertation was approved for the Ph.D. degree on 15 June 1922, and later that year his 1851 Exhibition Scholarship was converted to a senior scholarship. In 1921 he had made his first visit to the classic Norwegian localities, and to V. M. Goldschmidt’s superbly equipped laboratory in Oslo, which made a lasting impression on him. His Norwegian experience clearly influenced his seminal work on contact metamorphism in the Comrie area of Perthshire, Scotland (1924). Tilley’s important contribution here was the derivation of the possible assemblages of silica-poor hornfelses from the corresponding classes of Goldschmidt’s free silica hornfelses. Tilley showed that all the hornfelses could have formed from mixtures of sericite, chlorite, and quartz, the dominant minerals of the sediments from which they were derived. This was the most comprehensive study of a British thermal aureole yet made. Further papers followed on contact metamorphism, in one of which, in 1926, he recognized the significance of manganese in promoting the formation of garnet.

At the same time Tilley was working on the regionally metamorphosed rocks of the Start area of South Devon, England (1923). Here he discovered that the so-called Green Schists could be divided petrographically into two types, both having the chemical composition of basalt and representing different grades of metamorphism. He recorded for the first time that in the presence of a notable content of manganese, garnet develops before biotite in regional metamorphism.

In 1923 Tilley had met N. L. Bowen while on a field trip with Harker in Skye; Bowen became a close friend who was greatly to influence Tilley’s thinking. On 31 December 1923 he was appointed demonstrator in petrology for five years. At this time Tilley’s research was mainly concerned with regional metamorphism in the Scottish Highlands. G. Barrow’s pioneer map of the metamorphic zones of the Southeastern Highlands (1912) had been largely neglected, but Tilley extended it by publishing in 1925 a preliminary survey of the metamorphic zones of the Southern Highlands and in 1926 a discussion of the genesis of biotite, almandine, and staurolite in regional metamorphism. This was pioneer work on which little advance was made until the advent of the electron microprobe some thirty years later. Several summers were then devoted to detailed mapping with G. L. Elles in the attempt to demonstrate tectonic inversion of the metamorphic zonation; this work, published in 1930, has not found general acceptance. During this period he delineated the mineral assemblages developed in the Loch Tay limestone with progressive metamorphism through the chlorite, biotite, and almandine zones of the associated pelites (1927).

In 1926 Tilley undertook to prepare accounts of Australian igneous rocks for inclusion in T. W. E. David’s projected “Geology of the Commonwealth of Australia,” a monumental work that was never published. This work, as well as seven papers between 1923 and 1940 on Antarctic petrology, represent his continuing contact with the interest in his native Australia.

In the late 1920’s Tilley began his great work on the dolerite-chalk contact at Scawt Hill, Larne, Northern Ireland. Following his by now established practice, he made an extensive collection and published preliminary papers on melilite as a product of interaction of limestone and basaltic liquid (1929) and on the new minerals larnite (1929) and scawtite (1930). The substantive work appeared in 1931. Tilley identified an exogenous contact zone of silicates grading outward into a coarse marble and then into normal chalk. The contact zone was unique in its mineralogy with spurrite rock, larnite (±spurrite) rock, spurrite-gehlenite rock, and spurrite-gehlenite-merwinite-spinel (± larnite) rock. He concluded from the evidence of these assemblages that solutions emanating from the dolerite had enriched the contact zone in silicon and some other elements and had raised the temperature of the limestone envelope more rapidly than could have been achieved by conduction alone. Equally remarkable was the endogenous contact zone where the olivine dolerite magma had been contaminated by lime to yield, in the first stage, pyroxene-rich rocks and a residual magma enriched by plagioclase and lime; in the next stage, titanaugite rocks; then titanaugite-melilite-nepheline rocks; and, in the final stage, a melilite rock by interaction of lime with an alkali magma having the composition of titanaugite-nepheline rock.

The importance of this work was that it showed in detail for the first time how a basic alkali residuum could be produced from a doleritic liquid by absorption of lime. This was the mechanism proposed earlier by Daly for the normal production of basic alkali rocks; Tilley, however, drew attention to the very limited zone of contamination at Scawt Hill and to the dominance within it of pyroxene-rich dolerite and pyroxenite, forcing him to the correct conclusion that limestone assimilation has a restricted potential to generate alkali magmas. Detailed work on the mineralogy of Scawt Hill led to the discovery of six new minerals: larnite, Ca₂SiO₄ (1929); scawtite, Ca₇Si₆O₁₆CO₃(OH)₄ (1929); portlandite, Ca(OH)₂ (1933); hydrocalumite, Ca₂ Al(OH)₇. 3H₂O (with H. D. Megaw and M. H. Hey, 1934); rankinite, Ca₃Si₂O₇ (1942); and bredigite, α’-Ca₂SiO₄ (with H. C. G. Vincent, 1948). This was not only a study of signal significance in thermal metamorphism, it was also extraordinary in its wealth of fine detail. Later, in 1948, Tilley described the similar gabbro-limestone contact at Camus Mor, Muck, Scotland. In the light of these two examples, he discussed (1952) the trends of basaltic magma in limestone syntexis, showing that extreme iron enrichment is characteristic.

His appointment as lecturer in petrology in 1928 and his marriage settled Tilley in Cambridge. At this time a wind of change was beginning to blow through the Cambridge geology department. The Woodwardian professorship of geology, founded in 1727, had increasingly become identified with stratigraphy and paleontology. The professorship of mineralogy, founded in 1808, had gradually become identified with crystallography. The imminence of the simultaneous retirements of Hutchinson, professor of mineralogy, and of Harker, reader in petrology, prompted the university to consider establishing a department of minerology and petrology. Tilley took sabbatical leave while these proposals were being considered and worked at the Geophysical Laboratory in Washington, D.C., with J. F. Schairer on the system Na₂SiO₃-Na₂Si₂O₅-NaAlSiO₄, accomplishing also a formidable program of travel and fieldwork. Thus began a happy and lasting association with the Geophysical Laboratory. It was also his introduction to the United States, which he saw as a land of promise, where he was always stimulted, relaxed, and happy.

Tilley returned to Cambridge in late 1931 as the professor of mineralogy and petrology. Simultaneously his college, Emmanuel, elected him to a fellowship, which was to become an important factor in his life. A laboratory, built largely to his own design, was ready for occupation in 1933. His aim was to produce a laboratory of the first rank in teaching and research; in that he succeeded, with total dedication and some ruthlessness. He made it clear to his research students that the excellent optical, chemical, and X-ray crystallographic facilities that he had assembled were there to be used all of every weekday. He drove himself hard and expected his students to do likewise. He was never happier than when he was alone in the laboratory late at night or on weekends, sitting at his old Swift mincroscope, cigarette in his mouth and towel over his left shoulder ready for wiping the dust and ash from his slides. In the first decade a flood of papers came from Tilley and his students and, apart from the war years, that was to be the pattern for the future. He was elected a fellow of the Royal Society in 1938.

Once the laboratory was built, Tilley returned to a problem in contact metamorphism on which he had worked earlier with Sir John Flett. In 1930 they had concluded that the cordierite-anthophyllite hornfelses of Kenidjack, Cornwall, England, had been produced from the original greenstones with considerable loss of lime due to intensive weathering prior to metamorphism. In his new study (1935) Tilley advocated the hypothesis that the hornfelses had been produced by the metasomatic effect of solutions emanating from the nearby granite. In 1937 he showed that cordierite-anthophyllite granulites had been produced at the Lizard, Cornwall, variously by metasomatism of hornblende schists with introduction of silica and loss of lime during folding at high temperature and by isochemical metamorphism of sediments. The Lizard and Kenidjack rocks provide a splendid illustration of metamorphic convergence.

Work was continuing in Scotland: the rare rock eulysite (1936) and the rare mineral pyroxmangite (1937) were discovered at Glenelg, and mangniferous rocks were found at Gairloch (1938). There were two general papers on the paratenesis of kyanite-eclogites (1936) and kyanite-amphibolites (1937). These were all brief but significant papers.

Tilley’s last two papers on contact metamorphism, both of considerable importance, were published in 1951. The first, on the progressive thermal metamorphism of siliceous limestones and dolomites, extended the thirteen stages and ten mineral indicators of Bowen (1940) to seventeen stages and thirteen mineral indicators; tilleyite and rankinite were placed in the sequence and talc became the indicator of the first stage. The second paper concerned the zoned contact-skarns of Broadford, Skye, Scotland, which occur at the contact of granite with dolomite horizons of the Durness limestone and which have locally been affected by boron-fluorine metasomatism. A wealth of rare minerals occurs in the complex skarns, and Tilley discovered here a new mineral that he named harkerite. The para geneses of skarn zones are explored in detail and this paper, like those on Scawt Hill, illustrates Tilley’s remarkable eye for minerals, his great skill in identifying the rarer minerals, and his flair for discovering new minerals. Since much of Tilley’s work on contact metamorphism had concerned carbonate rocks, it was appropriate that a new mineral found in metamorphosed limestones at Crestmore, California, should have been named tilleyite in his honor in 1933.

Tilley’s Presidential Address to the Geological Society of London in 1950, “Some Aspects of Magmatic Evolution,” represented a change of course from metamorphic to igneous petrology that was to be permanent. Among those who knew him only from his publications, it evoked surprise; to his friends, colleagues and students it had long been evident from his lectures that his interests embraced all of petrology. The concept of magma types had been a principal result of the work of the Geological Survey on the Tertiary Igneous Rocks in Mull (1924) and in Ardnamurchan (1930). Subsequently the nature and genetic relations of primary and derivative basic and intermediate magma were widely discussed, but usually in a restricted context or in diffuse generalizations. “Olivine basalt” and “tholeiitic basalt” were terms in common use but there was little consensus as to their precise definitions. An authoritative and comprehensive survey was needed; it was provided brilliantly by the presidential address.

Few, if any, could match Tilley’s knowledge of the literature, his analytical power, or his ability to point the way forward. His definition of tholeiite and concept of the tholeiitic series gained general acceptance. His plea for more precise definitions and more quantitative chemical and mineralogical studies bore fruit; 1950 marked the start of a new and systematic approach in igneous petrology. Some of the questions posed had to wait many years for the development of high-pressure and high-temperature experimental techniques before they could be answered. Over the years the lucid, elegant style in which his early papers were written had given way, especially after Harker’s death in 1939, to a compact style making unusual demands of knowledge and acuity in the reader; the presidential address is a particularly difficult example of this new style.

Thirty years of driving himself to the limit in his research, coupled with the exacting duties in London necessitated by the coincidence of his presidency of the Geological Society (1949–1950), his presidency of the Mineralogical Society (1948–1951), and his vice presidency of the Royal Society (1949) took their toll in 1950. The preparation of a fitting presidential address to the Geological Society worried Tilley greatly and appears to have been the final factor in the nervous exhaustion that rendered him unable to deliver the address in person.

A brief convalescence in Canada, spent in filed work among the Haliburton-Bancroft alkaline gneisses, restored his health and started him on another significant problem, the petrogenesis of alkaline rocks. Characteristically he began with a close study of the critical mineral nepheline, made a global survey of the environmental controls on its composition in a wide variety of rocks, and concluded that whereas in the volcanic enviroment the compostion of the nepheline reflects that of the host rock, in metamorphic and plutonic occurrences the nepheline composition is restricted to the Moro-zewicz-Buerger convergence field. Tilley paid particular attention to hepheline-feldspar associations and showed that in the heteromorphic pair phonolite and nepheline gneiss, the feldspar phases become more sodic and the nepheline more potassic int he nepheline gnesis in response of changing temperature. In his paper on nepheline associations (1956) he advocated further experimental hydrothermal study of the nepheline-kalsilite-SiO₂ system by his former pupil W. S. Mackenzie and his associates at Manchester, which yielded a surer guide to the crystallization history of the undersaturated rocks. The alkaline rocks provided him with another new mineral, latiumite (with N. F. M. Henry, 1953).

Some seven years after his presidential address Tilley delivered the William Smith Lecture to the Geological Society of London in November 1957. He took as his subject “Problems of Alkali Rock Genesis”, was naturally very nervous about the quality of the lecture, and achieved another instant triumph. Some of the new data presented in the lecture related to a drill core from the Haliburton-Bancroft area of Ontario, Canada, which he had obtained with a view to investigating the problem of nephelinization; that he had had cut and examined 1,500 thin sections from the core is yet another reminder of the enthusiasm, energy, and stamina that he brought to each successive undertaking. Tilley demonstrated conclusively that the nephelinized rocks were original crystalline schists and gneisses converted to a subsolvus feldspathoidal assemblage by the action of metasomatic fluids derived from a feldspathoidal magma. Subsequently he showed with J. Gittins (1961) that the ultimate sources of the agents of nephelinization that had converted Grenville sediments into nepheline gneisses were hypersolvus nepheline syenites.

Tilley retired from the professorship of mineralogy and petrology at Cambridge on 31 September 1961 and was succeeded by W. A. Deer, a former pupil who had explored and interpreted the Skaergaard intrusion in east Greenland and was to be the principal author of a five-volume work on rock-forming minerals. Tilley’s final service to the laboratory he had created and equipped so well was the provision of electron-microprobe analysis by the appointment of J. V. P. Long in 1959. The year following his retirement was spent at the Geophysical Laboratory, where he had been appointed a research associate in 1956.

A visit to Hawaii had renewed Tilley’s interest in basalts, and he had started a program of experimental work with H. S. Yoder, Jr. (subsequently director of the Geophysical Laboratory), a remarkable collaboration between the supreme mineralogistpetrologist and the leading experimentalist. In 1962 they published a paper of almost 200 pages, “The Origin of Basalt Magma: An Experimental Study of Natural and Synthetic Rock Systems”, which was a triumph of interpretative petrology and experimental skill. The basis of their approach was to treat natural basalts and their chemical equivalent, eclogites, as bulk compositions in a multicomponent system the crystallization trends of which could be followed by the quenching method. They established clearly that the two basalt series could form in the region of magma generation from the same parental source, a garnet peridotite. They showed also that under hydrothermal conditions the stability field of basalt is drastically reduced by the development of amphibolite and were able to conclude that amphibole settling and resorption not only may operate to convert tholeiitic to alkali basalt liquids but also may effect the reverse change, depending on the composition of the amphibole at the liquidus. As a by-product they demonstrated that for all basalt compositions plagioclase is the first mineral to disappear completely with rising temperature at pressures above 1.5 kbar, suggesting a possible mode of genesis of certain types of anorthosite.

Between 1961 and 1967 Tilley was engaged not only in experimental work with Yoder in Washington, but also in detailed petrological studies of the differentiation of basalts and related rocks with I. D. Muir in Cambridge, in the course of which much new chemical data was presented in eight papers.

Tilley’s last visit to the Geophysical Laboratory in 1967 did not end his active interest in experimental petrology; for Washington he substituted Manchester. In five papers with R. N. Thompson, he described and explained the melting relations of lavas from Kilauea, Snake River, Réunion, the mid-Atlantic ridge, and Nyiragongo. The style of these papers is terse even for Tilley’s later period. These were the basic data papers, which, following his usual practice, were to be followed by a significant work of synthesis and review. But that was never to happen. He had suffered a mild thrombosis in 1970; such attacks occurred with increasing frequency over the next three years. He knew that he was slowing down but was determined to keep going to the very end. He had been invited as guest of honor to a dinner on 20 January 1973 in Emmanuel College that was to celebrate the centenary of the last day in the life of Adam Sedgwick, but felt too tired to attend. He died quietly by his fireside four days later.

Tilley was much honored in his lifetime. The Geological Society of London awarded him its Wollaston Fund in 1924, its Bigsby Medal in 1937, and its highest award, the Wollaston Medal, in 1960; he was president during the year 1949–1950 and William Smith Lecturer in 1957. He was president of the Mineralogical Society of Great Britain between 1948 and 1951and, unprecedentedly, again between 1957 and 1960. He was an honorary fellow of the Mineralogical Society of America, which awarded him its Roebling Medal in 1954. The Royal Society of London elected him to its fellowship in 1938, appointed him a vice president in 1949, and awarded him a Royal Medal in 1967. He was president of the International Mineralogical Association between 1964 and 1970, presiding over meetings in Cambridge in 1966 and in Tokyo and Kyoto in 1970. He received honorary doctorates from the universities of Manchester, England, and Sydney, Australia, and he was vice master of Emmanuel College, Cambridge, between 1952 and 1958.

The naming of the mineral tilleyite in his honor in 1933 was a particular source of pleasure to him. A source of even greater pleasure was the dedication of a special volume of Mineralogical Magazine, with papers by colleagues and former students, to mark his seventieth birthday; a copy sumptuously bound by Sydney Cockerell was presented to him at a dinner attended by many of the contributors at St. John’s College on 30 September 1965. A post-humous honor was the naming of the lecture theater in the department of mineralogy and petrology, where his softly booming voice had so often instructed and enthralled his audience over a span of thirty years, as the Tilley Lecture Theatre, a commemorative plaque of orbicular diorite being unveiled by H.S. Yoder, Jr. on 17 January 1981.

No petrologist of his generation ranged over so wide a field as Tilley; to significant contributions to the study of both contact and regional metamorphism must be added seminal works on the genesis of both basaltic and alkaline igneous rocks as well as substantial contributions to mineralogy and effective application of experimental petrology. He never published until he was satisfied that he could go no further toward the solution of the problem in hand or had solved it. Thorough collection in the field (by himself or by the few whose ability he trusted) was followed by exhaustive petrographic investigation and chemical analysis of selected rocks and of significant minerals (however difficult it might be to obtain the separations necessary before the advent of electron microprobe analysis) and supplemented by his comprehensive knowledge of the literature. His style of research and his obliteration of any demarcation between mineralogy and petrology were passed on to his many students and by them to a generation of petrologists worldwide. Passed on too was his aesthetic appreciation of rocks seen in thin section under the polarizing microscope.

BIBLIOGRAPHY

I. Original Works. Tilley wrote more than 120 original papers, of which the most significant are “The Petrology of the Metamorphosed Rocks of the Start Area”, in Quarterly Journal of the Geological Society, 79 (1923), 171–204; “Contact Metamorphism in the Comrie Area of the Perthshire Highlands”, ibid., 80 (1924), 21–71; “A Preliminary Survey of Metamorphic Zones in the Southern Highlands of Scotland”, ibid., 81 (1925), 100–112; “Some Mineralogical Transformations in Crystalline Schists”, in Mineralogical Magazine, 21 (1926), 34–36; “The Dolerite Chalk Contact at Scawt Hill, Co., Antrim. The Production of Basic Alkali Rocks by the Assimilation of Limestone by Basaltic Magma”, ibid., 22 (1931), 439–468; “Metasomatism Associated with the Greenstone-Hornfelses of Kenidjack and botallack, Cornwall”, ibid., 24 (1935), 181–202; “some Aspects of Magmatic Evolution”, in Quarterly Journal of the Geological Society, 106 (1950), 37–61; “ A Note on the Progressive Metamorphism of Siliceous Limestones and Dolomites”, in Geological Magazine, 87 (1951), 175–178; “The Zoned Contact- Skarns of the Broadford Area, Skye: A Study of Boron-Fluorine Metasomatism in Dolomites”, in Mineralogical Magazine, 29 (1951), 621–666; “Nepheline Associations”, in Verhandlingern van het Koninglijk Nederlands geologisch mijnbouwkundig genootschap, 16 (1956), 1–11; “Problems aof Alkali Rock Genesis”, in Quarterly Journal of the Geological Society, 113 (1958), 323–360; and “Origin of Basalt Magmas: An Experimental Study of Natural and Synthetic Rock Systems” in Journal of Petrology, 3 (1962), 342–532, written with H.s. Yoder, Jr.

II. Secondary Literature. The obiturary notices by G.A. Chinner in American Mineralogist, 59 (1974), 427–437, and by W.A. Deer and S.R. Nockolds in Biographical Memoirs of Fellows of the Royal Society, 20 (1974), 380–400, provide critical appraisals, bibliogrphies, and reminiscences.

Duncan McKie