110 PROCEEDINGS OP THE ACADEMY OF 



colored with that minuteness of detail attainable where a regular geological survey 

 has been made. It is therefore probable outliers of the Tertiary formations may 

 exist at a few points where the color would indicate the presence of the older 

 strata upon which they repose. It may also be the case that in some instan- 

 ces valleys have been excavated through the Tertiary so as to expose the 

 Cretaceous strata, at a few localities near the junction of those systems which 

 I have colored as Tertiary. Where any of the colors end abruptly against 

 black spaces, I merely mean to indicate that the formation is known to extend 

 that far, without pretending to say it may not occupy a greater area in that 

 direction.* 



As meagre as this map may appear, it is hoped it will not be considered 

 void of interest, when it is borne in mind that it represents the geology of 

 a country about much of which comparatively little accurate information has 

 been published, and the most conflicting opinions are entertained. As an 

 evidence of this, it is only necessary to state that in a comparatively recent 

 publication issued under the sanction of the Geological Society of France, 

 the author, with nearly all that had been published respecting the geology 

 of this country before him, colors almost the whole of this area, which 

 is now well known to be occupied by Cretaceous and Tertiary formations, as 

 Triassic and Jurassic. 



Respecting the section, it is perhaps only necessary for me to state, that so 

 far as it represents a profile of the country, it has been constructed from the 

 most reliable Barometrical measurements in the possession of the War Depart- 

 ment. The relative thickness of the various formations is given from careful 

 estimates, and not from actual measurements ; it is believed, however, that they 

 will not be found far from correct. 



As a brief history of former explorations in the North West, together with 

 descriptions of the new fossils, are given in another place in this number of 

 the Proceedings, I shall here confine myself to a few general remarks respect- 

 ing the geological formations represented in the map and section, and leave all 

 details for another occasion. 



The rocks of Nebraska Territory, so far as they have been determined, ap- 

 pear to be referrible to four of the great systems, viz. : — 



I. Carboniferous. III. Tertiary. 



II. Cretaceous. IV. Quaternary. 



That portion of Nebraska to which my attention has been directed, forms 

 the area or basin drained by the upper Missouri river and its tributaries, but 

 more immediately to the region bordering upon the Missouri itself, commenc- 

 ing at the mouth of the Platte river and proceeding thence to a jjoiut near the 

 base of the Rocky Mountains. 



System I. — Carboniferous. 



Ascending the Missouri river, we have the upper members of the Car- 

 boniferous system, or the Coal Measures, very well exposed at the mouth of the 

 Platte, and extending up the river about fifty miles, where they dip beneath the 

 water level of the Missouri. At Florence, seven miles above Omaha city, the 

 bed of the river is formed of these limestones, and this is the last exposure of 

 them which I observed in ascending the river. 



It is difficult to estimate with much accuracy the thickness of the different 

 beds, as the yellow marl or " Bluff Formation," a recent deposit, attains a 

 great thickness in this region, concealing the limestone except in a few lo- 

 calities. 



* By some mistake of the persons employed in New York to color the map, the little 

 Tertiary basin at the mouth of Judith River, and several small outliers of Tertiary near 

 White River, are colored a much deeper yellow than the deposits of that age on other 

 parts of the map. 



A very small exposure of Cretaceous rocks on the Yellowstone, just below the mouth 

 of Power River, was by a similar error left out. 



[May, 



