752 GEORGE DAVIS LOU DERBACK 



there patches of metamorphosed sediments. If the lavas coming up 

 through such rocks were chiefly rhyohtes or dacites, with perhaps 

 complementary types derivable from these by differentiation, they 

 might be considered to have been formed by melting of the granitic or- 

 granodioritic bedrock material ; but we find a great profusion of ande- 

 sites during the later Tertiary — hornblende andesites, pyroxene ande- 

 sites, hypersthene and other basic andesites (and latites), and finally 

 basalts rich in olivine. This phenomenon of basic rocks breaking 

 through and pouring out over granites is not uncommon in other regions. 



Can we reasonably imagine that in a great batholitic mass of gran- 

 ite, several or many hundred square miles in area, the granitic material 

 is only a mile or two thick, and there changes into basic diorite or 

 gabbro ? Many granitic areas occur in the western United States 

 through which basic lavas in abundance have broken, yet erosion, 

 which has in many cases deeply dissected such masses shows nowhere 

 such an internal structure. ^ 



Further information as to depth of focus may be derived from 

 sedimentary districts where, after the extrusion of lavas, the rocks 

 have been uphfted, tilted, and eroded. The thickness of sediments 

 necessarily traversed by a lava in rising to the surface at some definite 

 stratigraphic horizon can frequently be calculated closely enough for 

 our present purpose. In the Coast Ranges of California the late Ter- 

 tiary lavas have often had to make their way through several miles of 

 Mesozoic and Tertiary rocks to reach the surface. These volcanic 

 rocks do not show the characteristics of melted sediments; nor have 

 the exceedingly active earth processes at the end of the Tertiary, which 

 have uphfted and folded and eroded the rocks in such a way as to 

 expose frequently the whole Tertiary and Cretaceous series — not 

 rarely 20,000 to 30,000 feet in thickness^ — brought to light any of the 

 reservoirs formed by the melting of rock in situ. They frequently 

 testify to the fact that the sources were still lower, by the dikes found 

 cutting across the oldest layers. 



^ It is not to be denied that the later lavas breaking through earlier instrusives 

 frequently show striking chemical relationships, and this is true of the province 

 above referred to — but in general not such as could be explained by a remelting of the 

 granitic magna a short distance below the surface. The chemical relationships may 

 show consanguinity, but far from identity. 



2 These numbers are not sums of maxima — such would be much greater. 



