Vlerk, Isaäk Martinus Van Der

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(b. Utrecht, Netherlands, 31 January 1892; d. Leiden, Netherlands, 29 June 1974). stratigraphy, paleontology.

After completing primary and secondary education in his birthplace. Van der Vlerk studied geology at the University of Groningen (Netherlands) from 1914 to 1918 and continued his studies at the University of Basel (Switzerland). It was August Tobler, then in charge of the East Indian collections of the Basel Natural History Museum, who determined Van der Vlerk’s lifelong scientific interest. Guided by Tobler, a retired oil geologist, Van der Vlerk acquanited himself with foraminiferied paleontology. He returned to Groningen in 1920 to pass his final examination. He accepted a teaching position in biology at a grammar school, meanwhile continuing his work on the foraminifera in a collection of rock samples from Sumbawa (one of the Lesser Sunda Islands) lent to him by the Basel Museum. The results earned him a Ph.D. from the University of Leiden in 1922, with K. Martin as supervisor. His thesis clearly shows the feature that would characterize his work in the years to come: stratigraphical conclusions based on careful faunal analysis.

In the same year Van der Vlerk joined the Netherlands East Indies Mining Survery as a paleontologist in the exploration department. There he continued his studies of foraminiferied faunas, particularly aimed at establishing a stratigraphical scheme for the thick Tertiary succession. Eventually this led to the introduction, together with J. H. F. Umbgrove, of the letter classification. In its original form the letter classification was no more than an expedient to meet the needs of field geologists engaged in the systematic survey of the isles of Java and Sumatra. Six subdivisions based on larger foraminifera were distinguished, Tertiary a—f. In 1931 two additional units were introduced: Tertiary g and h, the latter roughly corresponding to pliocene. A further refinement was reached with additional numbers (Tertiary a1, a2, and so on). As long as correlationas with the European series and stages were not satisfatorily established, the letter classification fulfilled its purposes very well and remained widely used even after long-distance correlations based on planktonic foraminifera became avilable. Adams (1970) extended the letter classification to cover the entire tropical and subtropical Indo-West Pacific region.

In 1928 Van der Vlerk went to the University of Leiden as a curator of its geological collections and to lecture on paleontology and historical geology. His abilities were recognized by his promotion to a personal readership in 1931, to professor extraordinary in 1938, and finally to full professor in 1947.

During his early years in Leiden, Van der Vlerk’s interest remained in Tertiary larger foraminifera, although its scope widened to include the other end of the Tethyan realm, resulting in several publications on Central America and northern South America, mostly by his pupils. The finds of fossil mammalian remains at excavation sites for large construction works turned his interest to the thick Pleistocene succession of the Netherlands. His 1938 inaugural address as professor extraordinary on the Netherlands in the glacial epoch reflects this new interest. At first Van der Vlerk followed the classical subdivision of Penck and Brückner but soon became convinced that this scheme was unsatisfactory. In close collaboration with the paleobotanist F. Florschütz, he developed a regional subdivision in paleontologically characterized “stages” (1953). The principle of a regional clasification, independent of nay preconceived system of glacial-interglacial alternationa, proved extremely fertile when it was later shown that the number of paleontologically recognizable climatic oscillations was much larger than originally assumed by Van der Vlerk and Florschütz.

The fourteenth William Smith lecture, delivered to the Geological Society of London in 1959, marks another shift in Van der Vlerk’s scientific activity. In 1955 he had been charged with the directorship of the National Museum of Geology and Mineralogy at Leiden when it became a separate institution alongside the university’s geology department. Although retaining his chair, he left his teaching duties to a reader in the department and divided his time between making the museum a national institution and a renewed study of Tertiary foraminifera.

From then on, Van der Vlerk’s emphasis was upon detailed morphogenetic analysis of foraminifera. He retired early in 1961 from his chair and from the directorship in order to devote his remaining time and energy to this aspect of foraminiferid paleontology. In spite of failing health, he pursued his reserch, which led to the introduction of the “grade of enclosure” (1963), later replaced by the “degree of curvature” (1968). The latter is a more easily measured parameter of the same phenomenon, namely, the extent to which the second embryonic chamber (deuteroconch) encloses the first (protoconch) in the foraminiferid test. After comparing his results with those obtained with planktonic foraminifera, Vand der Vlerk considered the “degree of curvature” as a yardstick against which the geologic age of speciments could be measured. He illustrated this in a series of papers, the last one published only a few months before his death.

Apart from his original contributions to science, which were recognized by his election to membership in the Royal Netherlands Academy of Sciences in 1950, Van der Vlerk wrote books for a larger public and contributed a number of articles to encyclopedias. Although unpretentious and kindhearted, he demonstrated the courage of his convictions, as is shown by his uncompromising attitude toward the German occupiers during World War II. As a university teacher he disliked formal lectue courses to large audiences and was at his best in the more informal contact with small groups of graduate students.


I. Original Works. Van der Vlerk’s scientific papers through 1961 are listed in Evolutionary Trends in Foraminifera: A Collection of Papers Dedicated to I. M. van der Vlerk on the Occasion of His 70th Birthday, G.H.R. von Koenigswald et al., eds. (Amsterdam, 1963), 5–8. His later papers are given in Thomas van der Hammen, “In Memoriam Prof. Dr. I. M. van der Vlerk,” in Geologie en mijnbouw, 53 (1974), 241–243. His most significant papers include “Tertiaire gidsforaminiferen van Nederlandsch-Oost-Indië,” in Wetenschappelijke mededeelingen van de dienst van de mijnbouw in Nederlandsch-Oost-Indië, 6 (1927), 1–35, written with J. H. F. Umbgrove; “The Tertiary,” in Leidsche geologische mededeelingen, 5 (1931), 611–648, written with W. Leupold; “The Palaeontological Base of the Subdivision of the Pleistocene in the Netherlands,” in Verhandelingen der Koninklijke Nederlandsche akademie van wetenschappen, Afdeeling natuurkunde, eerste reeks, 20 , no. 2 (1953), 1–58, written with F. Florschütz; “Problems and Principles of Tertiary and Quaternary Stratigraphy” (fourteenth William Smith lecture), in Quarterly Journal of the Geological Society of London, 115 (1959), 49–63; “Biometric Research on Lepidocyclina,” in Micropaleontology, 9 (1963), 425–426; “Stratigraphie du Tertiaire des domaines indo-pacifique et mesogéen, esai de correlation,” in Proceedings of the Koninkijke Nederlandse Akademie van Wetenschappen, ser. B, 69 (1966), 336–344; “Oligo-Miocene Lepidocyclinas and Planktonic Foraminifera from East Java and Madura, Indoniesia,” ibid., ser. B, 70 (1967), 391–398, written with J. A. Postuma; “Two Methods of 334–338; “Evolution of an Embryo,” in Genetica, 39 (1968), 45–63, written with H. Gloor; “An Improved Method for Biometrical Research,” in Proceedings of the Koninkijke Nederlandse Akademie van Wetenschappen, ser. B, 76 (1973), 245–259.

II. Secondary Literature. In addition to the obituary by Thomas van der Hammen listed above, see R. Lagaaij, “Professor Van der Vlerk: An Appreciation,” in Evolutionary Trends in Foraminifera: A Collection of Papers Dedicated to I. M. van der Vlerk on the Occasion of His 70th Birthday, G. H. R. von Koenigswald et al., eds. (Amsterdam, 1963), 1–4; E. den Tex, “Levensbericht van Isaäk Martinus van der Vlerk,” in Akademie van wetenschappen, jaarboek 1974 (1975), 190–195. An account of Van der Vlerk’s role in the National Museum of Geology and Mineralogy is in G. E. de Groot, “Rijksmuseum van Geologie en Mineralogie 1878–1978, A Retrospect,” in Scripta geologicu, 48 (1978), 3–25. A review of the origin and subsequent development of the letter classification is given by C. G. Adams, “A Reconsideration of the East Indian Letter Classification of the Tertiary,” in Bulletin of the British Museum (Natural History), Geology, 19 (1970), 85–137.

Aart Brouwer