GEOLOGY 138 



Miocene as the case seems to be, there are some grounds for 

 believing that the extrusions of lava by the many volcanoes 

 of the western side of the Puerco valley, as well as those of 

 the western side of the Rio Grande valley, in roughly 

 continuous lines as they appear today, are due to the weak- 

 ening of the crust in each case along a major fault line and 

 the extrusions took place in the way, and with the relations, 

 which they sustain to each other and to the volcanic pheno- 

 mena of this region because of this faulting. In very few 

 places, and then in not any marked degree, is subsequent 

 faulting noticeable. This fact alone proves the faulting to 

 have been practically completed before the volcanoes and 

 lava flows took place. 



In several places along the Puerco valley there are good 

 evidences that the Tertiary sedimentaries were deposited 

 after considerable erosion had taken place over the Cre- 

 taceous, this erosion period being the equivalent of the 

 Araphoe and Denver transition period. 



There is a question bound to arise in the mind of him who 

 travels across the Chivote Mesa or in other words the im- 

 mense lava flow which extends northward from Mt. Taylor 

 seventy-live miles or more. 



This question suggests the idea that there was a dominat- 

 ing influence in the existing topography of the time which 

 invited the flow of Mt. Taylor lava to the northward, and, 

 since lava, as all other semi-liquids seeks the easiest routes 

 it seems reasonable to suppose that the Chivote Mesa of to- 

 day occupies an old river valley, probably that of the Puerco, 

 and as has been mentioned before, the crowding of the river 

 from its old pre-Mt. Taylor valley by this lava flow, coupled 

 with the upwarp along its head waters by the move- 

 ments which made the Continental Divide, are the two re- 

 sponsible factors in bringing the Puerco to its present dimi- 

 nutive size, which is so out of harmony with the size of its 

 valley and with the task of erosion which such a valley the 

 size of that of the Puerco represents. 



From a near view Mt. Taylor is quite disappointing. In 

 general outline the angle of the cone is low, but after the base 

 has been reached there still remains many weary miles of 

 hard climbing before the top is scaled. 



