OCCURRBISrCE AND SUCCESSION OF ROCKS. 195 



ference in chemical composition would almost inevitably accompany the 

 supposed separation by specitic gravity; and besides the porphyritical 

 crystals, other more minute solid particles would probably also sink, and 

 tend to the multiplication of centers of crystallization. 



Possibility of a metamorphic origin. — While the evideuccs of the cruptivo charac- 

 ter of this diorite are tolerably strong, they are not so conclusive as to 

 exclude a consideration of the possibility that the rock may be metamor[)hic. 

 As has been shown in Chapter III., one variety of the metamorphic diorite 

 is almost indistinguishable per se from the rock of Mount Davidson, and 

 another variety of the latter is distinctly brecciated. It is exceedingly diffi- 

 cult, if it is not in the present state of knowledge impossible, to comprehend 

 how the formation of pure and sharply developed crystals can go on in 

 media not sufficiently mobile to be regarded as fluid; yet we know that 

 tourmalines, garnets, and other minerals are sometimes beautifully developed 

 in metamorphic rocks, Avhich have not only retained their lamination, but 

 have offered an efficient resistance to the pressure of thousands of feet of 

 overlying strata. Most of the indications of the eruptive character of the 

 Mount Davidson and Cedar Hill diorite, taken singly, are thus not absolutely 

 incompatible with a metamorpliie origin. But until the origin of the granitoid 

 rocks has been more satisfactorily elucidated than heretofore, it is certainly 

 the duty of the geologist, while giving possible alternatives due weight, to 

 judge each occurrence on its own merits, and to seek explanations in compre- 

 hensible processes, rather than through unexplained analogies. At present an 

 eruptive origin can alone be regarded as probable for the Washoe diorites. 



Metamorphic diorite. — The grouuds for consIdcring the metamorphic diorite 

 as such, have already been given. It is a very puzzling rock in the field, 

 and may readily be mistaken in different occurrences for granite, diorite, 

 augite-andesite, or basalt. Wherever the underlying rock is exposed it is 

 sedimentary, except at tlie Wales Consolidated, where the nietamorphism has 

 penetrated to the underlying granite. It is also associated in the most inti- 

 mate way with the quartz-porphyry, and does not appear between the 

 stratified rocks and eruptive diorite. If the area occupied by the quartz- 

 porphyry were made continuous, it would completely cover all the meta- 

 morphic diorite in the District; and the evidence is tolerably strong that 



