324 1>. W. JOHNSON VOLCANIC NECKS 01'' MT. TAYLOR REGION 



tinned explosive ejection through the most favorable of these vents re- 

 sulted in their enlargement by the continuous detaching of fragment 

 after fragment from the sedimentary walls of the expanding tube or 

 chimney. Such a process would not tend to disturb the attitude of the 

 surrounding sediments. It is true that a mass of lava over a thousand 

 feet in diameter, thrust up bodily through horizontal sediments, could 

 hardly be expected to leave those sediments horizontal. In the gradual 

 enlargement of a vent from a few inches or feet to a thousand feet or 

 more in diameter, by the progressive sapping of the surrounding walls, 

 the sediments would be less apt to suffer disturbance, since fragments 

 from the walls of the conduit would break off more readily than wouhl 

 great masses of sediments be flexed upward by the locally applied force. 

 It is this latter process which appears to have been active in producing 

 the volcanic necks of the Mount Taylor region. The horizontality of the 

 beds associated with the Mount Taylor necks are therefore to be regarded 

 as a normal feature. 



Kesume 



In conclusion, it appears that the various jjhenomena associated witli 

 the buttes of the Mount Taylor region accord perfectly Avith that hypotli- 

 esis which interprets them as true volcanic necks, but do not admit of 

 their interpretation as remna:nts of flows, sills, or laccoliths. Vertical 

 columnar structure must be regarded as a normal feature in the upper 

 parts of volcanic necks, as must also the lack of disturbance noted in tlie 

 sediments about such necks. Inasmuch as the most conclusive evidence 

 on which a laccolithic origin was ascribed to the Devils tower of Wyoming 

 consisted of features which are very characteristic of the volcanic necks 

 of the Mount Taylor region, and since the descriptions of the tower 

 mention no features that might not occur in connection with, a volcanic 

 neck, the writer feels that the origin of the tower should be regarded 

 as an open question until further field study affords evidence upon 

 M'liich a decisive answer may be based. 



Eefeeences 



C. E. Button : Mount Taylor and the Zuui plateau. Si.xth xVnu. Rept. U. S. 



Geological Survey, pp. 106-198, 1884-1885. 

 T. A. Jaggar, Junior: The laccoliths of the Black hills. Twenty-first Ann. 



Rept. U. S. Geological Survey, pp. 163-290, 1899-1900. 

 J. S. Newberry : Report on the exploring expedition from Santa Fe to the 



junction of the Grand and Green rivers (by J. N. Macomb), with geological 



report by J, S. Newberry. Washington, 1876. 



