POSSIBLE INTERPRETATIONS 321 



bombs. Dense igneous rock is characteristic of intrusives, but the lava in 

 the Mount Taylor buttes is often vesicular, sometimes extremely so. 



Were laccoliths or sills so numerous in the region, we ought to find 

 some of them shown in section in the sides of Mount Taylor mesa; yet 

 nothing of this character has been observed there. On the other hand, 

 good sections of volcanic necks still connecting with overlying cinder 

 cones and lava flows have been described by Button, these necks bearing 

 every resemblance to tlie closely associated but more completely eroded 

 examples farther out from the mesa. 



In view df the above facts, it would seem inadmissible to assign a 

 laccolithic or sill origin to the Mount Taylor buttes ; hence we must con- 

 clude that vertical columnar structure and undisturbed associated sedi- 

 ments are not safe guides in distinguishing between remnants of laccoliths 

 or sills and volcanic necks. 



R. AS REMNANTS OF SURFACE FLOWS 



Can the Mount Taylor buttes be regarded as remnants of surface 

 flows? This would explain the vertical columnar structure and undis- 

 turbed sediments as well as the vesicular character of the lava in places. 



Again we meet the serious difficulty of being compelled to postulate 

 an erosion which Iiappened to leave several hundred more or less cylin- 

 drical towers of lava, but which failed to leave any larger remnants 

 which could be readily recog-nized as undoubted flows. The great lava 

 caps of Mount Taylor mesa and its outlying remnants, at a fairly uniform 

 and distinctly higher level, are obviously not to be considered in this 

 connection. The fact that no distinct lava-flow remnant has been ob- 

 served among the great number of Ijuttes seen in the erosion lowlands of 

 the region is sufficient to negative the hypothesis here considered. 



But there are other objections to the hypothesis. The buttes occur at 

 different elevations above the valley floor as well as at different strati- 

 graphic horizons. In order to interpret the buttes as remnants of flows, 

 we must assume numerous flows at different levels, or else a single flow 

 over a very uneven topography. But we have already seen that there is 

 no evidence of any volcanic activity in this part of the district later than 

 that which produced the great lava cap, and that this lava cap spread 

 out over a surface of relatively faint relief. 



Eemnants of surface flows should show liorizontal contacts with the 

 underlying sediments, and only rarely vertical contacts where the lava 

 flowed against some cliff or valley wall ; yet no horizontal contacts were 

 seen in connection with the Mount Taylor buttes, although the erosion 

 of ravines is often favorable for the disclosure of such contacts if they 



