DISSECTED VOLCANO OF CRANDALL BASIN. 6ll 



volcano of Crandall basin, may be inferred from a consideration 

 of the geological structure of this ancient volcano. The magmas 

 which solidified within that portion of the core now exposed, and 

 those in dikes within a radius of two miles, must "have occupied 

 positions at nearly the same distance beneath the surface of the 

 volcano, that is, at a depth of about 1 0,000 feet and over. The 

 one was as deep-seated or abysmal as the other, and yet their 

 degrees of crystallization range from glassy to coarsely granular. 



The influence of pressure on the crystallization is not recog- 

 nizable either in the size of grain or the phase of crystallization. 

 Marked changes in the crystallization may be traced horizon- 

 tally in the immediate vicinity of the core. They are rapid near 

 the core, and are accompanied by the induration and metamor- 

 phism of the surrounding rocks. They are in a general measure 

 independent of the size of the rock-body, since narrow dikes 

 within the core are coasely crystalline, while much broader ones 

 in the surrounding breccias are very fine grained. It was, unques- 

 tionably, the differences in the temperature of the core rocks and of 

 the outlying breccias which affected the degree of crystallization. 

 The former must have been more highly heated than the 

 latter rocks, and the magmas solidifying within them cooled 

 much slower than those injected into the outlying parts of the vol- 

 cano. In this case the depth at which the magmas solidified 

 appears to have been of little moment in comparison with the 

 temperature of the rocks by which they were surrounded. 



The core of gabbro and diorite with an intricate system of 

 veins of middle grained porphyritic rocks, and radiating dikes of 

 aphanitic and glassy lavas, encased in an accumulation of tuffs and 

 breccias with flows of massive lava, constitute an extinct or com- 

 pleted volcano. The central core consists of the magmas that 

 closed the conduit through which many of the eruptions 

 had reached the surface. In solidifying they became coarse 

 grained. The question naturally suggests itself, Are these coarse 

 grained rocks any less volcanic than those that reached the sur- 

 face ? What part of a volcano is non-volcanic ? 



Joseph P. Iddings. 



