312 D. W. JOHNSON A-()Lf'ANJO NECKS OK M'J'. TAYLOR REGION 



butte was no.t seen at close range, but tlie Bcdinients apjjear to continue 

 well up toward the top of the neck at the nortliern end, indicating a more 

 or less nearly vertical contact. 



jSTuinber 3 (plate 2T). This butte is rudely cylindrical in cross- 

 section, is fairly flat-topped, of smaller diameter than Cabezon, and does 

 not form so prominent a landmark as many of the others. A complete 

 circuit of this Imtte was made for the purpose of examining it carefully 

 on all sides. 



On the eastern side the yellow and brown Cretaceous shales and sand- 

 stones are M'ell exposed, and are seen to be quite horizontal and little 

 affected by heat. They may be traced to within less, than fifty feet of 

 the butte at this point, but the ])recise contact is obscured. All sedi- 

 ments have been stripped away from this side of the butte down to tlie 

 level of a hard sandstone layer, and this layer and its nnderlying beds 



FiGl'RK 4. — 'rwiii Peak, nhawiiiy liorizoiiia! tieilhiiviita irell iij) htiila of Butte. 



are seen to be horizontal so close to the butte wliich rises above them that 

 the appearance from a distance suggests a horizontal contact between 

 igneous rock resting directly u])on tJie sc<linients. A closer examination 

 shows that in no exposure does the igneous rock rest u])on the sediments; 

 but large remnants of tlie sediments found Avell up toward the summit of 

 this butte on other sides proves that the contact is essentially vertical. 



The internal structure of the butte is fairly typical of tlie greater 

 number of those examined. It is found to be composed of a ])eculiar 

 agglomerate consisting, of angular or somewhat rounded fragments of 

 vesicular, generally reddish lava, loosely held together, often containing 

 fragments of sandstone and shale, and shot through in all directions by 

 great tongues or stringers of more massive columnar lava. The columns 

 in. this butte are often finely developed at riglit angles to the contact with 

 the agglomerate, showing that the liquid lava was the last to be intruded. 

 Doubtless in many cases there have been several alternations of the two 

 types of activity. The agglomerate suggests a more or less explosive 



