562 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



acter. Tbis is distinctly noted in the descriptions just given, and in the 

 reports, where they are described in detail. 



3. There is a general resemblance in their rocks. 



4. They occur in regions of sedimentary rooks rather than in those 

 where the raetamorphic rocks prevail. 



They differ as follows : — 



1. The amount of disturbance in the sedimentaries caused by the 

 eruption of the igneous material differs very considerably. In the Elk 

 Mountains, the sedimentaries have been carried up, broken off, over- 

 turned, intersected by dikes, and forced apart by intrusive sheets. In 

 other areas, the erupted material appears to have forced its way through 

 the strata and spread out with scarcely any disturbance. Again, in 

 some localities, on reaching the shales, the igneous rock has pushed its 

 way between the layers, causing an uplift in the upper layers by the 

 wedge-like lateral intrusion, while the lower rocks are undisturbed. In 

 other cases, both the lower and upper strata are tipped up at the ends by 

 the passage of the igneous material through them. Sometimes, how- 

 ever, it appears to follow the planes of stratification instead of breaking 

 across. 



2. In some areas, the sedimentaries are highly' metamorphosed, and 

 in others seem to have been but little affected by any heat that may 

 have attended the eruption. 



3. Although there is a general resemblance in the rocks, individu- 

 ally they differ. Some of the groups are trachytic, others are apparently 

 granitic; in a, few, we seem to have diorite, and other areas combine 

 within their limits the different varieties. 



These differences are only in degree, and prove the propriety of class- 

 ing the different areas together. 



Although modified in several instances, the general plan appears to 

 be the same. The igneous material came up through fissures iu the 

 sedimentaries, sometimes tipping up their ends, and sometimes passing 

 through without disturbing them. On reaching the Cretaceous shales, 

 it generally spread out in them, and pushed into and across them 

 dikes and intrusive sheets of the same igneous rock. The elevation in 

 some cases appears to be due to the lateral intrusion, but in others, a 

 portion, at least, of the elevation is due to actual upheaval caused by 

 the eruptive force. The mountains as they now exist are doubtless 

 largely the result of erosion, the hard igneous rock opposing greater 

 resistance to erosive influences than do the surrounding soft sediment- 

 ary beds. 



Did the eruptive material originally reach the surface ? A study of 

 the southwestern areas would seem to give a negative answer to this 

 question ; but what is true in regard to particular areas cannot be nec- 

 essarily predicated of all the areas. If the mass of erui)tive material 

 did not come to the surface, it may have sent out dikes that did. In 

 Park View Mountain, the dikes occur in the lignitic beds. Perhaps, if 

 we could go far enough below the surface there, we might find a con- 



