THE CORE OF THE CRANDALL VOLCANO. 229 



introduced to emphasize the structure and indicate where the dikes would 

 have been found if there had been sufficient time to hunt them out. Slide 

 rock and forest obscure parts of the country, and parts of it have not been 

 visited, as the general character of the geology was recognizable from a 

 distance. From the data obtained there can be no reasonable question as 

 to the arrangement of the dikes. The great majority of those in Crandall 

 Basin radiate from the middle of Hurricane Mesa. A smaller number 

 radiate from a second center, 3 or 4 miles east of the first. 



It was during the study of the district, which was traversed along much 

 the same lines as those along which it has just been described, that the 

 conviction forced itself upon the writer that the locality toward which the 

 majority of dikes converged must have been the center of great volcanic 

 activity, and would prove to be the location of what was once the conduit 

 or throat of an ancient volcano, and might possibly exhibit rocks represent- 

 ing a coarsely crystalline development of the magmas which had filled the 

 dikes. It was, consequently, with great expectations that he led his pack 

 train over the uninviting and even forbidding country drained by Crandall 

 Creek, from Miller Creek across the densely timbered valley of Timber 

 Creek, and over the precipitous ridge into the bottom of Closed Creek ; and 

 having reached the gulches draining the suspected core, it was with great 

 satisfaction that he found himself surrounded by blocks of gabbros and 

 diorite of decidedly coarse grain. Here was in reality the core of an ancient 

 volcano, the conduit through which lava had risen to the surface, from 

 which it had escaped in lateral fisures through the surrounding rocks, and 

 in which it had eventually solidified. 



A description of this core necessarily involves a consideration of the 

 petrographical character of the rocks composing it, which in the case of. the 

 other rocks of the district has been deferred to a subsequent part of this 

 chapter; and in order to maintain a logical sequence in the study of all of 

 the rocks of this volcano the detailed description of the granular core will 

 be given in connection with the petrography of the rocks. A general 

 statement of its character, however, will be in place here. 



The round-topped spur between the twin gulches, frequently referred 

 to, consists of granular gabbro which grades into diorite. Grabbro also 

 forms the bottom of the gulches and extends up the flanks of the precipitous 

 spurs encircling the gulches. The greater part of the spur on the west 



