70 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



the formation of offshore deposits was marked by an assemblage of 

 fossils which survived the changing conditions and reappeared at 

 several horizons. 



It was to be expected that during the period of reconnaissance 

 surveys, coal groups belonging near the base of the Upper Cre- 

 taceous should be correlated with others elsewhere, which are in 

 highest formations of the series. One familiar with the facts, as 

 now understood, is not astonished by the contradictions, when he 

 considers the conditions under which the earlier work was done. 

 During recent years, detailed studies by geologists of the National 

 surveys of the United States and Canada have done so much toward 

 removal of uncertainties, that it is possible to present a comparative 

 table of formations, which, as a generalization, is near enough to the 

 truth for purposes of this study. ^^ 



The first systematic classification of the western Cretaceous 

 was presented by Hall and Meek.-'' Hall had financed an expedi- 

 tion to make collections between the Missouri River and the Mau- 

 vaises Terres, Meek being in charge. The succession, based chiefly 

 on Meek's observations, is 



Eocene, Tertiary Formation, clays and sandstone, etc., containing 



remains of mammalia, 250 feet. 

 Cretaceous Formation, 



5. Arenaceous clay, passing into argillaceous sandstone, 80 feet. 

 4. Plastic clay, with calcareous concretions containing numerous 

 fossils. This is the principal fossiliferous bed of the Cre- 

 taceous on the upper Missouri, 250 to 300 feet. 

 3. Calcareous marl, containing Ostrea congesta, scales of fish, 

 etc., 100 to 150 feet. 



-5 The writer would not neglect acknowledgment of his great indebted- 

 ness to the writings of W. T. Lee, T. W. Stanton, N. H. Darton, F. H. Knowl- 

 ton, E. Stebinger, R. W. Stone and W. R. Calvert, of the United States 

 Geological Survey and to those by D. B. Dowling, of the Geological Survey 

 of Canada. Several of these students have been unreserved in communi- 

 cating unpublished material ; but they must not be held responsible for con- 

 clusions offered by the writer, some of which may appear to them far from 

 correct. 



26 James Hall and F. B. Meek, "Descriptions of New Species of Fossils, 

 from the Cretaceous Formation of Nebraska," Mem. Amcr. Acad. Arts and 

 Sci., 1856, p. 405. 



