46 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 480 



Before 1920, Reinhardt Thiessen developed at the U.S. Bureau of Mines 

 a system of terminology based primarily on data derived from thin sections of 

 coal. Thiessen' s work was paleobotanically oriented and was undoubtedly influ- 

 enced by his close association with David White. Thiessen did not publish the 

 essentials of his methods. When Schopf joined the U. S. Bureau of Mines staff 

 in 1943, he was determined "to find out how Thiessen' s system of coal petrography 

 actually works. " After much research he did publish (1956) a coherent account of 

 the system. 



Since the late 1920s and early 1930s, most of the work in coal petrography 

 has been related to evaluating coal for various commercial uses. The terminology 

 of modern coal petrography is based on nomenclature originally developed by Stopes 

 (1919) from megascopic examination of coal. This system was soon adapted to 

 describing the coal elements revealed with a microscope. It is now commonly 

 known as the Stopes- Heerlen system because it was revised and presented at the 

 Second International Carboniferous Congress at Heerlen in 1935. Although the 

 system is fundamentally based on "macerals" (constituents of coal considered 

 analogous to minerals in rocks), the plant origin of the macerals has been gener- 

 ally recognized . The International Handbook of Coal Petrography (1963), prepared 

 by the International Committee for Coal Petrology, defines terms for coal constitu- 

 ents commonly used in coal petrography. Cady (1942) introduced the name "phy- 

 teral" for organic plant forms or fossils in coal to distinguish them from the organic 

 material of which the fossils may be composed (macerals) . Much of the work us- 

 ing the Stopes- Heerlen system has been done with reflected light (metallurgical 

 techniques), although thin sections have also been used. 



Cady (1939, in Lowry, 1945, 1960), Parks and O'Donnell (1956), Marshall 

 (1955), Schopf (1956), Harrison (1961), and others have discussed and compared the 

 Thiessen and Stopes- Heerlen systems of terminology. The relations of the two systems 

 to the paleobotanical composition of coal are readily recognized, although some 

 uncertainties have developed in their use where relations to paleobotany have not 

 been established. Thiessen (1913) briefly recounted the early history of the study 

 of microscopic coal constituents. Much of the subsequent history of coal petrol- 

 ogy, including contributions by Midwestern workers, is reported in Marshall(1955). 

 How the field of coal petrography has expanded was indicated by Berry, Cameron, 

 and Nandi (1967), who discussed the developments in coal petrography in North 

 America, and by Mackowskr (1967), who wrote of the recent progress in the field. 



Although many papers concerned with petrography of coals in the Illinois 

 Basin have been published, most of them are only indirectly related to paleobotany. 

 In addition to the summaries in Thiessen (1913) and Marshall (1955), reports on 

 coal petrography are listed in Willman et al. (1968), Nevers and Walker (1962), 

 and in the lists of publications of the Illinois, Indiana, and Kentucky Geological 

 Surveys. 



Probably the earliest micropetrographic study of Illinois Basin coals was 

 reported by Thiessen in 1913. In that classic contribution, The Origin of Coal, 

 White and Thiessen (1913, pis. 1-4, 43-54) provided exceptionally fine illustra- 

 tions of plant compressions on the bedding planes of bituminous coals from Illinois 

 and recognizable botanical components in sections of the Rock Island (No. 1) Coal 

 Member (which they called "Exeter Coal") and Shelbyville Coal Member of Illinois, 

 as well as of isolated spores and cuticles. The study, which was initiated to 

 determine the relations between plant composition and coal types and their proper- 

 ties, mentions some of the plants in the Rock Island and Shelbyville Coals. 



