POSSIBLE INTERPRETATIONS 323 



where the cooling effect of the sedimentary wall would exceed the cooling 

 effect of the more distant tipper air, resulting in a change from vertical 

 to radial columns. If the lava in the neck was continuous with a lava 

 cone above, both being highly heated and the cone having a crater in it 

 situated above the neck, there would result a vertical columnar structure 

 in the upper part of the neck, the columns gradually curving outward 

 and becoming radial farther down.* The vertical columnar structure 

 would extend much deeper in necks of large diameter than in necks of 

 small diameter. The buttes in the Mount Taylor region conform to 

 these expectations quite closely. Many of them show evidence of re- 

 peated eruptions, with irregular columnar structure in the different lava 

 tongues. Some are composed almost wholly of lava, with vertical colum- 

 nar structure in the upper part, the columns curving outward and be- 

 coming radial farther doAvn. The larger necks show the best vertical 

 columns, while the smaller necks quite uniformly show an irregular or 

 more radial arrangement. In many cases, of course, only the deeper 

 portions of the smaller necks are preserved, the higher portions standing 

 the best chance for preservation in the necks of large diameter. 



Volcanic necks, continuing indefinitely in depth, should be exposed at 

 any elevation or stratigraphic horizon below the former surface to which 

 erosion happened to reach. The buttes of the Mount Taylor region are 

 foimd at all levels, from tlie former upland surface dowu to the bottom 

 of the present valleys. 



The erosion of volcanic necks might occasionally show a limited liori- 

 zontal contact due to irregularity in the walls of the neck, but most of 

 the contacts exposed by erosion should be more or less vertical. All the 

 contacts observed about the Mount Taylor buttes were essentially vertical. 



Eegarding the attitude of the sediments enclosing the necks, it would 

 appear that they might reasonably be expected to remain practically 

 undisturbed, if we admit the gradual enlargement of the vents from 

 formerly insignificant apertures, as advocated by Button. Our own 

 observations led to the conclusion that there were fissures in the region, 

 many of which could not be detected without more detailed field study 

 than we could make, but a few of which were more noticeable because 

 they were opened and filled with lava, forming dikes which are still pre- 

 served, or else associated with differential movements developing faults 

 which are sometimes visible in the mesa walls. Along these fissures were 

 occasional vents, probably of exceedingly small diameter to begin with, 

 where escaping volcanic material found its way to the surface. Con- 



* I submitted the problem to Professoi- Wm. Hallock, of Columbia University, and am 

 Indebted to him for information on this point. 



