lif'fRODUCttOl^. 



as Tertiary, but no reference is made to it in the text, owing, as Prof. 

 Hall writes me, to some delay in the preparation of the map. 



He also says: 



" The geological formations and the boundaries between them were de- 

 termined from the best information I could derive from all sources within 

 my reach. 



" I had before me all the publications which had reference to any por- 

 tion of that country, and also the collections of rocks and fossils made bj^ 

 Dr. Parry and Mr. Arthur Schott. I had, moreover, the aid of Dr. 

 Parry in giving me his oral explanations of the country passed over and 

 examined during the exploration. There may have been other sources 

 which have now passed out of my mind, j * * * absorbed all in- 

 formation which came within my reach, and did the best I could to ex- 

 press it upon the geological map." 



Prof. Marsh, in his description of the Lake basins of the Rocky Moun- 

 tains, makes this statement regarding the Pliocene lake basins:* 



"At the close of the Miocene, a subsidence took place east of the 

 Rocky Mountains. A great Pliocene lake was thus formed directly over 

 the eastern Miocene basin, having nearly the same boundaries on the 

 north and west, but extending much further east, and stretching south 

 nearly to the Gulf of Mexico. It covered an area at least five times as 

 great as the older lake, while its deposits attained a thickness of nearly 

 or quite 1500 feet. This lake basin may with great propriety be called 

 the Niobrara basin, since the Niobrara River cuts through its typical 

 strata for more than 200 miles of its course. 



" The beds in this basin lie nearly horizontal. They are light in color, 

 and much more arenaceous than the Miocene below. The upper strata 

 consist of hard sandstones or calcareous grits, which weather but slowly, 

 and hence still form the great table lands over much of the area of the 

 basin. * * * 



" The fauna of this lake basin indicates a warm temperate climate. 

 The more common mammals are a mastodon, rhinoceroses, camels and 

 horses, the latter being especially abundant." 



In accordance with this description of the Pliocene lake basins by Prof. 

 Marsh, Prof. J. LeConte, in his "Elements of Geology," maps the Nio- 

 brara basin as extending southward over the area occupied by the Llano 

 Estacado.t 



* American Journal of Science, vol. IX, January, 1875, p. 52. 

 t Map of Tertiary Times, showing outlines of coast and places of principal 

 Tertiary lakes. Elements of Geology. J. LeConte. 

 This statement is based on recent advice from Prof. LeConte. 



