20 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The distinction between Upper and Lower Cretaceous in the United States is 

 based on the natural divisions which are regarded as characteristic of North Amer- 

 ican Cretaceous. The Mexican Survey recognizes a threefold division, Eo-, Meso-, 

 and Neo-Cretaceous, which is adjusted to European standards but which can not 

 yet be carried out on the geologic map of Mexico, because not enough detailed work 

 has been done in that country. The Lower Cretaceous is most widely distributed 

 in Mexico, and the areas which are colored accordingly include also those of the 

 Upper Cretaceous. 



The line of division between the Cretaceous and Tertiary is everywhere in 

 doubt from South America to Canada. The positions of the Laramie and its sup- 

 posed equivalents and of those formations which have been placed in the Shoshone 

 group of Cross have been controverted and are still matters of opinion. The data 

 for the map assembled by the most recent work of the United States Geological 

 Survey in Utah, Colorado, Wyoming, and Montana were compiled by Willis, who 

 placed the Laramie in the Cretaceous, in accordance with the decision of the Survey, 

 and assigned the Shoshone group to the "Paleocene," after Scott. "Paleocene" 

 has not, however, been accepted by the Survey's committee on geologic names, 

 and the group is here described as "earliest Tertiary or latest Cretaceous." The 

 Canadian Survey stated that the corresponding line in western Canada should be 

 drawn at the base of the Edmonton. 



Many other questions will present themselves to the student, who is referred 

 to the text for the data on which the map is constructed. 



As this work includes no chapters describing the igneous rocks, figures 1 and 2, 

 showing the distribution, as indicated on the geologic map, of the post-Cambrian 

 intrusive rocks and the Tertiary and later effusive rocks, respectively, are inserted 

 here. With the post-Cambrian intrusive rocks are included extrusive rocks of the 

 same age, and with the Tertiary effusives are included associated intrusive rocks. 



ACKNOWLEDGMENTS. 



The chapters have been read by geologists of the United States Geological 

 Survey and revised in accordance with their suggestions. The writer owes special 

 acknowledgment to Messrs. C. D. Walcott, Charles Schuchert, E. O. Ulrich, and 

 A. W. Grabau, who have read the citations on the lower Paleozoic; to Mr. E. M. 

 Kindle, who read the Silurian and Devonian chapters in the original and also in 

 the revised drafts; to Messrs. G. H. Girty, David White, and G. H. Ashley, for 

 criticism of the Carboniferous chapters; to Mr. J. Perrin Smith, for revision of the 

 Triassic chapter; to Mr. T. W. Stanton, for revision of the Jurassic and Cretaceous 

 chapters; to Mr. F. H. Knowlton, for comments on Mesozoic paleobotany; to Mr. 

 W. H. Dall, for consideration of the several chapters on the Tertiary; and to Mr. 

 T. Wayland Vaughan, for contributions to the discussion of the Tertiary of the 

 Atlantic, Gulf, and West Indian provinces. 



The suggestions made by these able critics have been followed so far as the 

 scope of the work permitted, but the responsibiUty for the substance and form 

 of the text rests none the less with the compiler. 



The preparation of the manuscript geologic map has been difficult and tedious. 

 It is a task requiring peculiar skill and experience to prepare suitable copy of so 



