EROSION OF JURASSIC LAND. 273 



range not far north of Santa Fe, and thence ran northwestward across the 

 Rio Grande valley, westward around the head of the present basin of the 

 San Juan river, and again northward across the west flanks of the San 

 Juan mountains at the head of the Dolores and San Miguel rivers, turning 

 eastward again across the heads of the Uncoinpahgre and other tributaries 

 of the Gunnison. 



It is possible that the northwestern extension of the Jurassic land-mass 

 connected with the southern end of the Sawatch island, for all Mesozoic 

 sediments are now wanting between the Arkansas and Gunnison rivers. 



The San Juan area was, during the period of elevation, uplifted and eroded 

 in such a manner that along the northwestern flanks the Jura-Dakota beds, 

 which were deposited during the succeeding subsidence, not only rested in 

 distinct angular unconformity upon the edges of the Triassic and upper 

 Carboniferous beds, but overlapped in places onto the underlying lower 

 Palaeozoic series. On the southern flanks, however, the angular uncon- 

 formity is not readily apparent, but the Triassic beds apparently thin out 

 and finally disappear to the eastward of the Animas canon, having probably 

 been eroded away. 



Sawatch Island. — The area of the Sawatch island was very largely increased 

 during this movement, not only by the recession of the surrounding seas, but 

 by the actual addition of adjoining areas by dynamic movements. That oil 

 its northern extremity has already been mentioned. The uplift of the 

 northern portion of the Mosquito range and of the Gore mountains extended 

 its area to the borders of the Middle park. A thickness of not less than 

 6,000 feet of beds has been eroded from the crest of the Mosquito range, and, 

 although it cannot be assumed that this was entirely accomplished during 

 the period of elevation, it is evident that enough time must have elapsed to 

 allow of the complete denudation of the northeastern flanks of the Mosquito 

 range where Jura-Dakota beds now rest directly upon the Arcluean. 



On the west side of the Sawatch there is more definite evidence of the 

 amount of erosion that must have taken place after the upheaval that 

 accompanied this movement. It is in the Elk mountains that this record is 

 now found — a region that was so intensely disturbed in the post-Cretaceous 

 movement that it is now impossible to correctly outliue the land area that 

 was added to the Sawatch island, or even to say with certainty that the por- 

 tions of this region that must have been above water were actually connected 

 with it. It is probable, however, that a ridge extended eastward from the 

 region at the head of the valley of Roaring fork to Treasury mountain, and 

 that another extended southward toward the ancient land-mass at the head 

 of the Gunnison valley, from each of which the Triassic beds, and in some 

 cases a large portion of the upper Carboniferous, were eroded. The best 

 localities for studying the effects of this erosion and the unconformity of 



