about the head of the Missouri, é. 239 
well marked Cretaceous beds pass up quite imperceptibly into an 
enormous thickness of Lignite Tertiary. Passing over the divi- 
ding crest to the head waters of the Missouri, we did not observe 
any indications of Cretaceous rocks until we had descended be- 
low the three forks, where we find traces left after erosion. The 
do not reveal themselves conspicuously until we arrive within: 
twenty or thirty miles of Fort Benton where the black plastic 
clay begins to overlap the Jurassic rocks with its characteristic 
fossils, and on reaching Fort Benton the plastic clay is quite ho- 
mogeneous and is developed to a thickness of eight hundred feet. 
As we proceed toward the mouth of the Judith River and near 
the Judith mountains we find quite thick beds of coneretionary 
sandstone which form the “Stone Walls,” “Citadel,” &. Iti 
from these beds that we have obtained a group of fossils which we 
have referred provisionally to No. 1, but which seem to be spe- 
cifically distinct from all others in the West. It may be that when 
this group of beds now referred to Nos. 1 and 2, comprising a 
thickness of fifteen hundred to two thousand feet in this region 
are more carefully studied that several subdivisions will be made, 
having equal importance with the others. Durin the past sea- 
Son our route led us along the ‘divide’ between the Missouri and 
ellow Stone rivers south of the Judith mountains, so that we 
passed outside of any good exposures of No. 1, as well as beyond 
the limits of the estuary beds at the mouth of the Judith. We 
Must await a more thorough and detailed exploration of this re- 
ag we can state with entire confidence the succession of 
e beds. 
VIE. Terrrary Deposrrs. 
In speaking of the Tertiary deposits of the Northwest, so far as 
known at the present time I propose to separate them into four 
divisions which will be sufficient for our immediate oses 
Ist, Estuary Deposits. 2nd, True Lignite Beds. 38rd, Wind - 
River Valley Deposits. 4th, White River Tertiary Deposits. 
J 
Ing character. “ Opinions of a somewhat conflicting nature have 
been entertained in regard to them, owing to the peculiar char- 
that the 
fossils, A thin 
