F. GEOGRAPHY. 271 



velopnient of soft sandstones and marls. The identification 

 rests upon the analogy of position and lithology. The "Red 

 Beds" are massive sandstones and conglomerates as usual. 



The only important mountains are the Sierra La Plata. 

 They lie toward the northeast, and are principally of carbon- 

 iferous rocks, so highly metamorphosed as to have lost all 

 apparent structure. A large number of rich lodes of gold 

 and silver have been recently discovered in this group about 

 the sources of the Rio La Plata, and an extensive placer bai- 

 ls located near its exit from the mountains. 



In the extreme northeast corner of this district there is a 

 group of trachytic buttes, including Lone Cone, which be- 

 long to the San Miguel Mountains. \Vest of the Mesa Verde, 

 almost in the centre of the district, stands the " Late " group, 

 of which Ute Peak is the culminating summit. It covers an 

 area of some forty square miles, and is simply a mass of tra- 

 chyte pushed up through and poured out over the floor of 

 the Dakota group. 



In the extreme southwest corner, principally in Arizona, 

 are the Sierra Carisso, identical with the "Late" in nearly 

 every respect, differing only in having carried up portions 

 of the carboniferous rocks about their base, while a fragment 

 of the same beds is caught up in the centre of the group. 



Of the 6000 square miles, 5700 are of sedimentary rocks. 

 Two hundred and thirty of these, in the southeast, are of 

 the so-called lignitic ; S00, chiefly included in the Mesa 

 Verde, belong to the upper cretaceous; and the remaining 

 4900 to the lower cretaceous, and such of the earlier periods 

 as are exposed in the crooked and narrow valleys, and about 

 the trachytic groups. In the cretaceous series Mr. Holmes 

 examined a number of seams of workable coal, procured fos- 

 sils in ten distinct horizons, and expects to be able to iden- 

 tify these horizons with such corresponding ones on the At- 

 lantic slope. The section obtained is the most complete and 

 satisfactory made in Colorado up to this time. The trachyte 

 areas include about 250 square miles, and seem to present no 

 remarkable or unusual features. 



The prehistoric remains in the caiions and lowlands of the 

 southwest are of great interest, and the study of them by Mr. 

 Holmes was as complete as possible under the circumstances. 

 Many cliff-houses, built in extraordinary situations, and still 



