THE S-FOLD A PREVAILING STRUCTURAL TYPE. 271 



river. The character of this uplift was not the simple uptilting of a block of 

 the earth's crust into a monocline, as has been shown to be the prevailing 

 character of movement in the Plateau region by the geologists who have 

 worked there, nor the vertical upthrust of a block bounded by two lines of 

 faults, which one of them has propounded as the type of the uplift of the 

 Park province or Rocky Mountain region. It was the result of compress- 

 ive folding, producing a fracturing or faulting along the steeper side of a 

 one-sided or S-fold, which is the prevailing structural type in this region. 

 From the northern end of the Mosquito range and the Gore mountains, 

 thus raised above the ocean level, the sedimentary beds from Cambrian up 

 to Triassic, which had been deposited upon them around the northern end of 

 the Sawatch uplift, were almost entirely eroded away, a few patches only 

 remaining on the crest and steeper western side of the uplift to prove the 

 character of the fold. Around the eastern and northern flanks of this 

 uplift, from the waters which during the succeeding depression entered the 

 Middle park, whether from the north through North park or from the west 

 across the Park range north of the Gore mountains, the Jura-Dakota beds 

 were deposited directly upon the denuded Archaean ; west of the Park range 

 they stretched continuously across the fault line and rested in apparent con- 

 formity upon the Triassic beds, north of Eagle river and west of the fault 

 line, which had escaped erosion. 



This view of the structure of the region, which involves important modi- 

 fications in the structural history of the Mosquito range given in my mono- 

 graph upon the Leadville region, has naturally been adopted with extreme 

 reluctance and under the influence of gradually accumulating evidence in its 

 favor, combined with an inability to explain the known geological occur- 

 rences in any other way. In that monograph* I assumed, in the absence 

 of any direct evidence of dynamic movements previous to the close of the 

 Cretaceous, that the folding and faulting of the Mosquito range was probably 

 post-Cretaceous, although I foresaw the possibility and even probability that 

 further investigation might lead to a modification of this view. The age of 

 the porphyries, which were folded and faulted with the enclosing sedimentary 

 beds and hence were necessarily older than the dynamic movement, I as- 

 sumed to be late Cretaceous, since similar rocks are found in other parts of 

 the Rocky Mountains cutting through the latest Cretaceous formations. 



According to my present view a part at least of the uplift of the Mosquito 

 range must have occurred in Jurassic time, though I still think that the 

 mountains were further disturbed and uplifted during the great post-Creta- 

 ceous movement. The greater part, if not all, of the porphyries must, how- 

 ever, have been intruded before the Jurassic movement, and the original 



* 1886, pp. 23 and 31. 

 XXXVI— Bull. Geol. Soc. Am.. Vol. 1, 1889. 



