42 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



2. Several layers of hard limestone, very compact with Crinoids, 

 Corals, Chonetes mucronata, Athyris suhtilita, Productus, Sec; six feet. 



1. Greenish-yellow clay, underneath the most valuable and massive 

 bed of limestone, as showu in the illustration ; twenty inches thick. 

 Below this there is a layer of yellow limestone eighteen inches thick. 



Bed 2 in the section is the one that produces the valuable rock for 

 building purposes. The organic remains determine at once the geolog- 

 ical position of the rocks. 



About six miles above the mouth of the Platte I observed a large 

 number of bowlders or erratic rocks scattered over the hills, composed 

 of granite and red quartzite. These were undoubtedly transported 

 hither by glacial action ; and the rocks themselves come from the north 

 and northwest — from Dakota, Minnesota, and perhaps from the region 

 of Lake Superior, where the rocks abound. jSIear this point, also, a ledge 

 of rusty sandstone of Cretaceous age was seen capping the hills. Its 

 character has been described before, as a dark, ferruginous, coarse- 

 grained micaceous sandstone, but sometimes becoming a tough, close- 

 grained, compact, siliceous rock, or quartzite. It is very difficult to find 

 rocks of this group resting directly upon the beds below, from the fact 

 that in almost all cases a grassy slope intervenes, and it became a matter 

 of much importance to find the junction of the two great formations, or 

 ascertain what beds come between. 



In 1857, while making an exploration of this region, I was so fortu- 

 nate as to discover this apposition of the two formations, and the results 

 were published in a memoir in the Transactions of the American Philo- 

 sophical Society in 1862. The section taken at that time was observed 

 near the old Otoe Village, about eight miles above the mouth of the 

 Platte Eiver. 



The Cretaceous rock set directly u[)on the limestone, although we know 

 what a vast thickness of beds of various ages are absent. This illus- 

 trates what Professor Eogers has denominated, in his Geology of the 

 State of Pennsylvania, an unconformable sequence of beds ; that is, the 

 eye will observe no apparent want of conformity, the lowest bed of one 

 Ibrmation reposing upon the highest of the other, as if no interval had 

 occurred during the deposition. The section, in descending order, is as 

 follows : 



1. Gray, compact, siliceous rock, passing down into a coarse conglom- 

 erate, an aggregation of water-worn pebbles, cemented with angular 

 grains of quartz; then a coarse-grained micgiceous sandstone — twenty- 

 five feet. 



2. Yellow and light-gray limestone of the Coal-Measures, containing 

 numerous fossils — Spirifer cameratus, Athyris suhtilita, Fusulina cylind- 

 rica, with abundant fragments of coral and crinoid remains— twenty to 

 fifty feet. A, quartz rock ; B, conglomerate; C, coarse micaceous sand- 

 stone; D, carboniferous limestone. 



This conjunction of the two great formations at this point is quite in- 

 structive. We see the tremendous effects of erosion prior to the deposi- 

 tion of the sandstones, in the i'aat that hundreds of feet of limestones 

 must have been swept away. 



In Kansas, near Port Eiley, there are several hundred feet of Permian 

 and Permo Carboniferous rocks, not a trace of which can be seen in this 

 valley. Even in the Salt Creek Valley, above Lancaster, there is one 

 hundred feet or more of rocks that do not appear here; and yet I can 

 see no good reason for not supposing that all these rocks were deposited 

 here in the great oceans of the Coal period, but have been worn away 



