CHEMICAL INDICATIONS. 135 



silicificatiou followed serpentinization, while it is certain that seipentiniza- 

 tion postdated the formation of the holocrystallino metamorpliics. The 

 serpentinoid rooks, like the jihthanites, are sometimes intersected by quartz 

 veins, but the conversion of serpentine into opal has taken place only 

 locally and is for the most part referable, not to the Post-Neocomian epoch 

 of nietamorphism, but to the volcanic period of ore generation. 



A difficulty must always arise in discussing nietamorphic rocks, from 

 the inevitable lack of positive knowledge as to the composition of the sedi- 

 ments prior to nietamorphism. It is indeed one of the advantages which 

 the Coast Ranges aflfoi'd for the study of metamorphism that the origin of 

 the sediments is known and that the composition of the unaltered strata as 

 a whole is uniform. No geologist needs to be told, however, that this uni- 

 formity cannot extend to hand specimens. It is impossible to say that any 

 particular sample of pseudodiabase or serpentine once had the composition 

 of a second sample representing unaltered sandstone, and it is consequently 

 also impossible to ascertain the exact quantity and quality of the changes 

 which have been wrought in it by the action of mineral solutions. It fol- 

 lows, however, from the study of the relations of man}' metamorphosed 

 masses to the unaltered or slightly altered rocks surrounding them, that the 

 pseudodiabase, pseudodiorite, and serpentine are as a whole derived from 

 sandstones of average character. 



The fresh sandstones carry magnesia, but not in great quantities, for 

 the allothigenetic, ferromagnesian silicates form a small part of the mass, 

 while the matrix is sometimes nearly pure calcium carbonate and never 

 appears to contain considerable quantities of magnesia. The observations 

 cannot be reconciled without supposing that very large quantities of mag- 

 nesia have been supplied in solution from extraneous sources, though the 

 precise quantity cannot be determined in any given case. One and only 

 one evident source for this supply of magnesia exists, viz, the ferromag- 

 nesian silicates of the underlying granite. The wide horizontal extension 

 of the granite is well established. Its depth is entirely unknown, but must 

 be very great, since it is nowhere cut through. An inexhaustible supply 

 of mao-nesia was thus at hand, as well as heated waters, with a probable 

 upward tendencv at the period of metamorphism, Init under what precise 



