320 D. W. JOHNSON- — VOLCANIC NECKS OJ? MT. TAYLOR REGION 



with undisturbed sediments. There is also noted a tendency for the col- 

 umns to turn outward toward the base in the case of the Mount Taylor 

 examples, another feature which allies them to the Devils tower. Ag- 

 glomerate is abundant in the Mount Taylor examples, and is found in the 

 lower part of the Devils tower. There are certainly features of strong 

 resemblance between the buttes in the two places. In pointing out this 

 resemblance, I accept Professor Jaggar's photographic and written de- 

 scriptions of the Wyoming example, as I have not seen the tower. 



To such an interpretation for the Mount Taylor buttes there appear to 

 be serious objections. First, we should have to imagine hundreds of lacco- 

 liths or sills developing at all possible horizons in a series of sandstones 

 and shales, either near the surface, which was at the same time covered 

 with a flow of the same lava, or else in depth with long subsequent erqsion 

 and finally a surface flow similar to the material previously intruded. 



We should have to grant, further, that erosion at various levels and at 

 numerous places happened to be so nicely adjusted in every case that the 

 overlying arched beds were invariably completely removed, leaving only 

 the horizontal beds beneath, and that in some remarkable manner ero- 

 sion always removed just enough of the intrusive mass itself to leave a 

 rudely cylindrical tower, oval or circular in cross-section, resembling a 

 volcanic neck. If the buttes are remnants of either laccoliths or sills, 

 surely in all the examples studied there should be at least one which 

 would show some trace of overlying sediments; one in some other stage 

 of erosion than the penultimate; one which in some way bore distinct 

 resemblance to a laccolith or sill. 



Maturely eroded laccoliths or sills should show traces of the horizontal 

 contacts between the base of the intrusive mass and the underlying 

 sediments. The contacts seen in the Mount Taylor region were all verti- 

 cal. It is true that the horizontal contact might be wholly concealed in 

 one or even several remnants of laccoliths or sills, and that vertical con- 

 tacts might occasionally be seen in such remnants where the intrusive 

 mass cut across the bedding ; but it is beyond probability that in so large 

 a number of examples as have been studied in the Mount Taylor region 

 there should be found no trace of horizontal contacts, but many vertical 

 contacts, ii the buttes were really remnants of laccoliths or sills. 



The structure of the buttes makes the laccolithic or sill hypothesis 

 inadmissible. They are sometimes composed wholly of an agglomerate 

 indicating explosive activity, and at other times have numerous lava tubes 

 running up through the agglomerate, showing repeated uprisings of the 

 lava toward the surface. The agglomerate often contains good volcanic 



