M. T. Hill — Texas Section of American Cretaceous. 287 



A1„0 3 - 20-9 



Si0 2 . 41-1 



H 2 2-5 



CaO ...30-7 



B O 4-8 



100-0 

 While these results differ from those deduced by Kammelsberg 

 from his formula, they agree very well with all the results of 

 analysis No. 2. 



Although the Stromeyer and Marignac methods of estimating 

 boric acid have been shown to be unreliable, and the determi- 

 nation of any constituent of a mineral by difference is unsatis- 

 factory, nevertheless it will be seen, on comparing the results 

 of the various analyses, that the directly determined per- 

 centages of boric acid do not differ greatly from the older 

 values. 



I have to express my sincere thanks to Professor F. W. 

 Clarke, by whose kindness the material for these analyses was 

 put at my disposal, and also to Mr. Washington for kindly fur- 

 nishing the description of the colemanite. 



Laboratory of U. S. Geological Survey, 

 Washington, D. C, June 8, 1887. 



,^ 



, ^Xrt. XXXIV. — The Texas Section of the American Cretaceous ; 

 & by Kobert T. Hill. 



[Published by permission of the Director of the U. S. Geological Survey.] 



It is necessary to make a brief preliminary explanation of 

 the Cretaceous of the United States as it is generally under- 

 stood through accepted publications. There are at least five 

 widely separated areas where the system has been indepen- 

 dently studied by different authors, but little or no work has 

 been done to trace the direct stratigraphic relations between 

 them. In each of these areas, except the Calif ornian which 

 will not be touched in this paper, the North Atlantic (includ- 

 ing New Jersey, Maryland, District of Columbia, Virginia and 

 the Carolinas), the Gulf (including Alabama, Georgia, Mis- 

 sissippi, Tennessee, Arkansas and eastern third of Texas), 

 the Eocky Mountain (including the Trans-Pecos and lower Eio 

 Grande region of Texas), the principal subdivisions recognized 

 are those of the following table.* 



There is evident from a careful study of the faunas and 

 stratigraphy of these regions, throughout them all, within the 



*In the following diagram the solid lines represent complete faunal breaks, and 

 the dotted lines indicate subdivisions whose species range into the including 

 formations. 



