Maech 24, 1899.] 



SCIENCE. 



437 



and the transitions are so gradual, that it is 

 by no means easy to define the outlines of 

 the intrusive masses without personal in- 

 spection. Although never having been 

 followed as a continuous body, owing to 

 the nature of the topography, the zone of 

 induration is one of the marked features of 

 the region, and under favorable conditions 

 may be traced in the canyon walls for fifty 

 miles, with a width in places of more than 

 one-half mile. Another important and 

 significant feature is the inclination of the 

 breccias away from some well-defined axis 

 or central ridge. They do not as a rule 

 arch over any single powerful protrusion, 

 but present every indication of a broad 

 anticlinal structure, with the piled-up lavas 

 inclined toward the west and southwest on 

 one side and toward the east and northeast 

 on the other. Between the more massive 

 bodies that have been forced upward to 

 elevations above the general level there 

 may be found areas of indurated breccia, 

 traversed by a labyrinth of dikes and veins 

 in their efforts to force their way upward. 

 Without entering into petrographic de- 

 tails, a few words in addition to what has 

 already been said seem necessary. Granites 

 and diorites are seldom met with other 

 than in connection with the large uniform 

 stocks. As most of these stocks are only 

 partiallj"^ exposed, their volume can only be 

 a matter of conjecture, but in all the larger 

 bodies, such as Needle Mountain, the rock 

 is essentially that of a medium-grained 

 diorite or diorite-porphyry. A true granitic 

 structure is by no means i;ncommon. Most 

 of the powerful intrusions, as regards their 

 crj'stalline structure, may be classed as 

 granular. The great bulk of these crystal- 

 line rocks apparently carry some little 

 gronndmass. Porphj'ritic structure, with 

 little groundmass, is a characteristic feature, 

 with transitions into andesite-porphyrj' and 

 andesite. Manj' similar bodies of indefinite 

 outline, only partially exposed by erosion 



of the canyon walls, are andesites. Indeed, 

 all the relatively small bodies are andesitic 

 in habit, and the same is true of the many 

 outlj'ing bodies away from the general north- 

 west-southeast trend of the intruded rocks. 

 A field study of these rocks of vary- 

 ing degrees of crystallization shows clearly 

 that they were all exposed to virtually the 

 same degree of pressure of overlying rock, 

 and that their structural differences were 

 not dependent primarily upon pressure 

 from above. Many of these andesitic 

 masses are much smaller than the diorite 

 bodies and occur at much lower levels below 

 the superimposed load. All observations 

 upon the geological relations of these in- 

 trusives to the breccias tend to show that 

 their structural differences are dependent 

 far more upon the chilling effect of the sur- 

 rounding rock and the rate of cooling than 

 upon the pressure of the overlying rock. 

 Geologists and petrographers have been for 

 a long time investigating the structural 

 differences and mineral variations of igneous 

 rocks. Of these philosophical investigators 

 Professor Iddings stands in the foremost 

 rank. In an exhaustive petrographic study 

 of the Crandall Basin intrusive body and 

 its complex system of radial dikes of vary- 

 ing composition he reaches the conclusion 

 that they have all been derived from the 

 same parent molten magma, but crystal- 

 lized under different conditions. With this 

 conclusion I heartily agree. Dr. Jaggar, 

 who has been at work upon a petrograph- 

 ical study of the intrusive rocks of the rest 

 of the Absaroka Eange, has reached a 

 similar conclusion as regards the Ishawooa 

 intrusive stock and associated sheets and 

 dikes, and believes that they were derived 

 from a common molten magma, which is 

 quite in accord with geological observations 

 in the field. From these observations, 

 thus briefly and imperfectly stated, the 

 conclusion seems inevitable that the Isha- 

 wooa intrusive, for its entire length of fifty 



