404 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



gle Chain known as (he Park range; opposite the South Park this is 

 represented by the double elevation of the Mosquito and Bawatch 

 ranges, which in earlier geological time formed a single massive, but 

 are now separated by the great meridional valley of the Upper Arkan- 

 sas. To the south these two ranges coalesce in the single ridge of the 

 Sangre de OristO range, which trends tirst to the southeast, dividing 

 Wet .Mountain valley from San Luis park, then southward forming the 

 eastern front of the whole group, and finally disappears beneath the 

 desert plains of New Mexico. 



The westernmost tier of mountain uplifts, which is still more broken 

 and irregular, is constituted by the White Mountain plateau, on the 

 north, opposite the Park range; the complicated and lofty group of the 

 Elk mountains, which adjoin the western Hanks of the Sawatch uplift; 

 and to the south, separated from the latter by the broad mesa slopes 

 of the (iunnison valley, are the San Juan mountains, stretching west- 

 ward from the San Luis park to the Uncoinpaghre plateau. 



With the exception of Middle and San Luis parks, all the interior 

 valleys send their waters across or around the component parts of the 

 Front range into the Mississippi valley; the outlet of San Luis park 

 is southward through Xew Mexico into the Golf of Mexico; Middle 

 park is drained by the (hand river, which threads its way in deep 

 narrow gorges, between the White River plateau on the north and the 

 Sawatch and Elk mountains on the south, to the Colorado river. 



The geological structure of these different mountain masses is, as a 

 rule, very complicated, but as the study of the region progresses it 

 becomes more and more evident that its broad general features must 

 have been outlined at very early dates in its geological history. In 

 each of the great mountain masses are found representatives of the 

 Archean, Paleozoic, and Mesozoic groups. In but few have Algonkian 

 beds yet been recognized. Eruptive rocks of different ages are found 

 in all, but the greater manifestations of eruptive activity, in Tertiary 

 times, have occurred mainly in the southern and western portions of 

 the region. 



Five great transgressions, indicating as many orographic 5960 move- 

 ments, have thus far been recognized, and future study may disclose 

 others. These are: The post- Archean, the post-Algonkian, the late- 

 Paleozoic, the late- Jurassic, the post-Cretaceous. Minor movements 

 undoubtedly occurred in Tertiary times, as in the Wasatch region, whose 

 general effect has been to raise the whole group, and to compensate 

 for the degradation produced by erosion. 



In the middle belt, which is traversed by the party and may be taken 

 as a type of the structure of the whole group, the central uplift is 

 formed by the Sawatch massive, a great oval area of Archean rocks, 

 surrounded by an almost completely encircling fringe of Paleozoic and 



