318 D. AV. JOHNSON VOLCANIC NECKS OF MT. TAYLOK REGION 



Numbers 15 and 16. About a mile nortli of Great neck is a small 

 butte which appears to be composed wholly of agglomerate, although its 

 western side was not seen. One-tliird of a mile northwest of Great 

 neck is another small butte composed of both agglomerate and beauti- 

 fulh' columnar basalt, the columns being irregularly disposed. 



Number 17, Great neck (plate 30). With the exception of Cabezon, 

 this is the fmest butte in the region. According to Button's report, it 

 rises 1,800 or 1,900 feet above the valley and has a diameter of about 

 1,300 feet. Although that writer was unable to visit it, he reproduces a 

 picture of it in his report. 



On examination we found the greater part of this butte to consist of 

 columnar basalt. As seen from the northwest, the columns in the main 

 portion of the butte are vertical. Farther around to the west side the 

 colunms are vertical above, but curve outward toward the horizontal 

 below. In other places the columns are more irregular. On the north 

 side of the butte agglomerate and irregular basaltic columns are seen. 



The summit of the butte presents features of some interest. There 

 appear to be two horizontal layers of igneous rock lying on the columnar 

 portion of the butte and forming a sort of double cap for it. Ascending 

 the west side as far as the contact between the columns and the l)ottom 

 of the lower layer, I was able to see that the latter consisted of a more or 

 less decomposed, structureless sheet of lava, while above it came a thick 

 layer of agglomerate full of good volcanic bombs and exceedingly vesicu- 

 lar lava. These features suggest that the upper part of this great butte 

 represents its upward continuation, possibly into an overlying volcanic 

 crater where successive layers of material spread out in the bowl of the 

 ei'ater, to be subsequently preserved in part, because of their position 

 immediately above the resistant mass of the butte itself. In this connec- 

 tion it is noted that Great neck rises distinctly above the general level of 

 Prieta mesa just east, although it is lower tlian Mount Taylor mesa to 

 the west. 



Other buttes. From near the summit of the west side of Great neck, 

 one can count from 30 to 35 volcanic buttes, exclusive of the very small 

 ones of those already mentioned. Button estimates that in the Mount 

 Taylor region there are probably several hundred of these buttes exposed. 

 Some of these he reports as being exposed in the sides of the great lava- 

 capped mesa which formerly covered the Puerco A'alley region, a com-' 

 plcte section of a butte connecting with overlying cone and lava flow 

 sometimes being secured. Several of the buttes which we saw from a 

 distance appeared to lie but partly exhumed by erosion along the side of 

 the mesa, but we were unable to visit tliem to determine their exact rela- 



