304 D. W. JOHNSON VOLCANIC NECKS OF MT. TAYLOR REGION 



The desire to visit the district was strengthened by the tendency on 

 the part of some observers to regard vertical columnar structure and 

 undisturbed, surrounding sediments as features not to be expected in 

 volcanic necks, but rather as indicative of some other origin for buttes 

 which exhibit such features. Thus the Devils tower, in Wyoming, 

 has been referred by Professor T. A. Jaggar, Junior, to a laccolithic 

 origin partly because it rises above undisturbed sediments, and its beauti- 

 ful columnar structure is more or less nearly vertical. In a recent text- 

 book, "Elements of Geology," by Professor W. H. Norton, it is implied 

 (page 276) that vertical columns would not be found in a volcanic neck. 

 Dutton states that vertical columns are found in many of the i\Iounl 

 Taylor buttes, and rather implies that they are surrounded by undis- 

 turbed sediments. It was believed that a careful stud}^ of the buttes iu 

 the liio Puerco valley, with possible alternative interpretations in mind, 

 might serve to determine wliich featured are characteristic of volcanic 

 necks, which of remnants of laccoliths or columnar sheets of lava, and 

 which are common to both topographic forms. 



An opportunity to visit the region was offered in the spring of 190(), 

 during a geological excursion through parts of New Mexico, Arizona, 

 and Utah. The excursion, which included a M-agon trip of over 1,500 

 miles during a vacation of four months, was made possible by appro- 

 priations from Harvard University and the Massachusetts Institute, of 

 Technology and private gifts from Mr George G. Crocker, Senior, and 

 other friends of the Institute. Ten days were spent in a trip to the 

 Mount Taylor region, where tlie principal buttes between Cabezon and a 

 point a few miles north of Juan Tafoya were studied and the data pre- 

 sented below were collected. The district included between these two 

 Mexican villages was selected because it contains, according to Dutton, 

 the largest and best preserved buttes in the Mount Taylor region. 



I was accompanied in the field by Dr H'. W. Shimer, of the Massa- 

 chusetts Institute of Technology, who made a study of the stratigraphy 

 of the Puerco valley, the results of which will soon be published. The 

 pliotographs reproduced on plates 27 and 39 were taken by Doctor 

 Shimer. I am indebted to Professors W. M. Davis, J. B. Woodworth, 

 and T. A. ,Jaggar, Junior, for reading the manuscript of this paper and 

 ofTerino- ei'itieisms and suo^gestions. 



Purpose of the Investigation 



Briefly stated, tlie objects of the trip- were as follows: 

 (1) To ascertain fui-ther details regarding the structure of the sup- 

 posed necks and tbeir relation to the surrounding sediments. 



