ERUPTIVE KOCKS. 301 



the acid types of the two classes, and hornblende-porphyrite and hyper- 

 sthene-andesite among the more basic, are almost identical in chemical com- 

 position. 1 The loci of eruption in either case are not more than ten miles 

 apart, and yet in one instance the molten material congealed at a depth of 

 over ten thousand feet and in the other at the very surface ; and the result- 

 ing rocks are distinct varieties, differing more in the case of the basic ones, 

 where composition is more closely alike, than in the acid. 



In a discussion of the origin of a certain group of laccolites, an argu- 

 ment has been made in favor of the theory that their laccolitic or intrusive 

 character is dependent on the density of the eruptive magma (which is neces- 

 sarily a function of its chemical composition) ; that the molten mass would 

 stop in its upward progress through the sedimentary strata, when it had 

 reached a point at which the average density of the rocks below it was 

 greater than, and that of the rocks above it less than, its own average 

 density. This argument is, however, materially weakened by the instability 

 of some of the premises. First, it is assumed that the density of laccolitic or 

 intrusive rocks is less than that of erupted lavas. Even should this prove 

 to be true of that group, it would not be a sufficiently wide basis on which 

 to found a generalization for laccolitic or intrusive bodies as a whole. Sec- 

 ondly, the data used in support of a necessary condition of this argument, 

 namely, that "the acidic rock of the laccolites must have been heavier in 

 its molten condition than the more basic rock of the neighboring volcanos," 

 are, as the author acknowledges, insufficient, even if trustworthy. Aside 

 from the value of this argument as such, however, the facts observed in this 

 region, as mentioned above, afford a direct proof of observation against it. 

 Moreover, as no chemical analyses of the rocks of this jrroup of laccolites 

 were made, it is by no means impossible that they are, as a class, much 

 less acid than the author supposed. 



Whether or not there was a difference in the impelling force in the case 

 of intrusive sheets and of surface flows is a purely speculative ques- 

 tion, for which no direct evidence can be obtained. It might perhaps be 

 argued that, if the magma from which each of these series was formed 

 originated at essentially the same position within the eaith's crust, it would 



'See analyses 1 and 9 and 5 ami 10, Table I, Appendix H. 



