PURiPOSE OF THE INVESTIGATION 305 



(2) To see if it were possible to harmonize the facts o^bserved in the 

 field with some other theory of origin than that elaborated by Button. 



(3) To determine as far as possible what features may be used as 

 critical evidence in discriminating between volcanic necks and remnants 

 of eroded laccoliths or columnar sheets of lava. 



The Conclusions reached 



The studies led to several conclusions which are stated more fully in 

 subsequent pages, but which may here be summarized as follows : 



(1) Those buttes of the Mount Taylor region observed by us are nn- 

 doubted volcanic necks, as determined by Button, and do not acUnit of 

 any other interpretation. 



(2) A'ertical columnar structure is apt to characterize the upper por- 

 tions of volcanic necks, as should be expected from tlieoretical consider- 

 ations and as is fully exemplified in many of the Mount Taylor necks. 

 Such structure, therefore, can not be used as evidence in discriminating 

 between necks, and remnants of laccoliths or columnar lava sheets. 



(3) Frequent sections about many of the necks show them to be sur- 

 roiaided by horizontal sediments which have not been disturbed by the 

 intrusi(m of the necks. Disturbed sediments, therefore, are not a neces- 

 sary accompaniment of volcanic necks, and the absence of such disturb- 

 ance can not be urged as an evidence of some other origin for any bulte 

 whose natiire is in doubt. 



(4) Vertical colunms wliicli curve outward at the base, as in tlie Devils 

 tower and in some of the Mount Taylor necks, are more apt to occur in 

 volcanic necks than in renmants of laccoliths or columnar sheets of lava. 



(5) In general, it appears that the selection of any specific structural 

 feature as a guide to the critical distinction between volcanic necks and 

 remnants of laccoliths or columnar sheets of lava is unsafe. There are 

 few, if any, features Avhich might not occur, to some extent at least, in 

 buttes having different origins. The concordant testimony of a variety 

 of evidence from a large number of different buttes, however, can hardly 

 leave any doubt as to their origin, especially wlien the general geological 

 history of the region is taken into account. 



Literature 



The only detailed account of the buttes of the Mount Taylor region 

 is that by Dutton in bis paper, "Mount Taylor and the Zuni plateau." 

 The paper discusses several problems of wide scope, the volcanic buttes 

 being but one of the interesting features considered. The fifteen pages 



