THE ERUPTIVE ROCKS. 309 



Up to this point the description covers an area of unbroken granitic 

 rocks of various types, supei'ficially separated by shallow bridges of schist. 

 Like the roots of a great tree inverted, there radiate from this central mass 

 numberless dikes of every size, the connection being proved in many cases 

 and probable in all. These dikes are of two kinds, the fine an<l uniform 

 grained biotite- (or two-mica-) granites or granitite, and the coarse to very 

 coarse muscovite-granite veins — pegmatite. The former are generally, the 

 latter sometimes, interbedded in the schists for long distances. Toward the 

 periphery of the area the pegmatite dikes cany secondary veins of albitic 

 granite with many rare minerals. Within the area cut by the dikes and 

 suiTounding it in a broad halo the country rock is filled with quartz veins 

 and pegmatite lenses of every size, derived, I doubt not, from the gi-anite. 



On the eastern side of the river there stretches north from the Bel- 

 chertown tonalite ("syenite") area a region where the schists are so crowded 

 with jjegmatite veins that they (the schists) sink into unimportance. This 

 continues across Amherst, and in Leverett is followed by a large area of 

 almost unbroken granite. 



The discovery and description of the peculiar type of eruptive masses 

 to which the name "laccolites"'has been given by Gilbert — great mushroom- 

 like bodies of lava thrust up into the bedded rock to a certain level and then, 

 expanding into a cake-like mass between the beds, pushing up the superin- 

 cumbent strata into a low dome, but not reaching the surface — suggested to 

 E. 8uess" the name "batholites" for the .similar but more extensive masses of 

 granite which occupy a position in the crystalline schists analogous to that 

 of the laccolites in the newer rocks. It is in this connection that the obser- 

 vation of Hitchcock is interesting, that the great masses of granite seem to 

 be set free by the denudation of the' schists above them, and the further 

 observation — which I have had occasion to make repeatedlv — that where 

 the schists are so cut up by the interlacing granite dikes that the latter 

 make up far the greater portion of the sui-face, and even where long isolated 

 sheets stand vertical or nearly so in the great granite masses, the prevalent 

 strike and dip of the surrounding schists are strictly maintained, indicating 



' Gr. K. Gilbert, Kept. Geol. Henry Mountains. A. C. Pe.ile, On a peculiar type of er>iptive 

 mountains in Colorado: Bull. U. S. Geol. and Geog. Siirv. Terr., No. 3, p. 551. F. M. Endlich, Erup- 

 tive rocks of Colorado: Tenth Ann. Rept. U. S. Geol. and Geog. Surv. Terr., p. 199. 



■ E. Suess, Das Antlitz der Erde, p. 219. 



