114 EXPLORING EXPEDITION PBOM SANTA 11, 



central portion of ihe continent, throwing down its sediments onlyalongthe shores and 

 up tlie valley of the Mississippi; the inland Miocene-Tertiary having been deposited in 

 bodies of fresh water. It is possible, however, that some portion of this series is of 

 Eocene age, though of this we have as yet no proof. 



It is true that, in Texas, the Eocene beds, distinctly marked by their fossils are 

 lithologically similar to some of those under consideration, and they are there entirely 

 conformable to the Cretaceous strata, of which they seem but a continuation: the two 

 formations blending and not separable by any sharp lines of demarcation. 



NACIMIENTO FOUNTAIN. 



Though not a lofty or extensive mountain range, this is perhaps the most instruct- 

 ive and interesting of all those which I had an opportunity of examining in the western 



country. Its extreme altitude is about 10,000 feet; its length something like 50 miles. 

 Throughout this distance it forms a Bingle simple ridge of nearly uniform height, with 

 no peaks nor depressions. At cither end it gradually falls off, and is lost in the level 

 of the plateau country which nearly surrounds it. Although its physical features arc 

 so unpretending, Its geological structure is in the highest degree interesting and sug- 

 gestive; such indeed that it seems to me it not only furnishes a key to the mode of 

 formation of all the great ranges of the Rocky Mountain system to which it belongs, 

 but that, if properly studied, it would serve to explain nearly all the difficulties of 

 that now much-mooted subject, the origin of mountain chain-. What its structure is 

 may be very briefly told. 



The central core or axis of the Nacimiento Mountain is composed of massive red 

 granite, similar to that so common in the other ranges of the Rocky Mountain system. 

 This forms its summit and the greater part of its mass. Upon the slope of the granite 

 axis rests the Carboniferous formation, for the most part limestone, in main' places 

 nearly vertical, yet hut slightly metamorphosed. ( hitside of the Carboniferous are the 

 white and red sandstones, marls, and gypsums of the Trias; many of these beds also 

 standing quite vertical, but wholly unchanged. Outside of the Trias the Lower Cre- 

 taceous rocks form another distinct circle; beyond these, the Middle Cretaceous shales; 

 still beyond, the great group of Upper Cretaceous sandstones and marls, which 1 have 

 just described. 



When Ave stood on the summit of the mountain all these different formations 

 were spread out before as as on a geological map, each distinguishable by its color or 

 texture, and as readily recognized as though traced on a diagram for a lecture-room. 



The impression produced upon my mind by our first ascent of the Sierra do Naci 

 miento may l>e gathered from the following extracts from my notes: 



"After arriving in camp (50) i went over with Mr. Fisher to the Xacimiciito 

 Mountain, which we ascended. Our approach was up a beautiful but narrow valley, 

 through which a clear, cold stream flows for some distance, then sinks and rises at 

 intervals, forming a series of fine springs. The vegetation of this valley is almost 

 identical with that of the valleys of our northern route; the elevation being nearly the 

 same — 7,500 feet. The timber is yellow pine, pifion, cedar, (two species,) hitter and 

 sweet cotton-wood, GambelPs oak, and the narrow-leaved willow.* On the summit of 



* rimta ponda-osa, P. edulis, Juniperua tetragona, J. rtrginiana, PopuUts angustifolia, I', mohillfera, Queroua Gombdli, 



Salix angustifolia. 



