68 Upper Cretdceous Seacoast in Montana [266 



greater concentration of the waters. Considering this fact, 

 then, the waters should show a decrease rather than an in- 

 crease of mineral matter with depth, if the waters were of 

 surface origin. Which adds more evidence to the above, that 

 these natural brines are of connate origin. 



AN UPPER CRETACEOUS SEACOAST IN MONTANA 1 



By W. T. THOM, JR. 



INTRODUCTION 



The nonpersistence of lithologic units is the greatest ob- 

 stacle to systematic stratigraphic work and correlation. Par- 

 ticularly is this true when it is necessary to determine the 

 relationship of marine and continental deposits. That this 

 difficulty may be largely overcome by a correct " paleophysio- 

 graphic " perspective of the period dealt with is now becoming 

 the accepted doctrine ; and the writer has undertaken this 

 sketch both as an illustration of the way in which ancient 

 physiographic conditions may be deciphered and as a demon- 

 stration of the close genetic relationship of some of the differ- 

 ent lithologic phases of the Judith River formation. 



The writer studied this formation in the area lying north 

 of the Yellowstone Biver, near Billings, Mont., while serving 

 as an assistant to Eugene T. Hancock, of the United States 

 Geological Survey, and it is with his very kind consent that 

 this article is published. The accompanying sketch map will 

 serve as a geographic guide to the reader in following the 

 discussion. 



STRATIGRAPHIC GEOLOGY 



The later Upper Cretaceous sediments of the Montana 

 group are exposed throughout this region, forming about 90 



1 Published by permission of the Director of the United States 

 Geological Survey. 



