GENERAL GEOLOGY 



309 



which represents only a portion of its former greater 

 extent. The buttes occur in the eroded area. 



Tlie numerous vents from which the lava spread out 

 to form the great surface cap must have connected with 

 some kind of conduits extending downward tlirough 

 the sandstone to the depths from vdiich the lava rose. 

 As soon as the resistant cap was removed, the under- 

 lying sandstones would be quickly worn away ; but here 

 and there ought to be left standing some masses of re- 

 sistant igneous rock representing the lava which hard- 

 ened in the conduits. A view across the Puerco valley 

 reveals many shafts or cylindrical masses of lava rising 

 as buttes well above the valley lowland, produced by 

 erosion of the softer beds. The conclusion is most 

 natural that the buttes represent "necks" of lava whicli 

 hardened in tube-like conduits leading from unknown 

 depths up to vents on the former surface, the surface, 

 together with the cones and flows which were built 

 upon it, having been removed liereabout by erosion. 

 Such a conclusion is strengthened if, like Button, one 

 sees sections of such shafts of lava in the side of the 

 mesa eoiinecting above with cones and lava flows not 

 yet wholly destroyed. He does not doubt that under 

 the portion of the lava cap which still remains are 

 many other sucli "necks" leading np to the cinder cones 

 and other vents still seen on tlie surface of the cap, 

 whicli will some day be revealed ])y the removal of the 

 cap and the enclosing sediments. 



It is essential to the better appreciation of what is to 

 follow that the three main points in the history of the 

 IMount Taylor region, as noted above, be ke]it in mind. 

 Tliose are, briefly, (1) the reduction of the region to a 

 peneplain; (2) the occurrence of a period of volcanic 

 activity during which the peneplain was covered with 

 a lava cap derived from numerous vents; (8) the ex- 

 tensive erosion of the region, including the removal of 

 the lava cap and underlying sediments from large areas, 

 and the exposure of the shafts of lava which hardened 

 in the conduits. 



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