PEOOF OF METAMORrHISM. l^Jj 



make any investigation of tlie nietamorphism very profitable. The sides 

 show that a portion of tliese i-ocks are tliorougbly recrystalHzed and that in 

 mineral composition and in structure they strongly resemble the metamor- 

 phosed rocks of the Knoxville group in the Coast Ranges. There is at 

 present nothing improbable in the supposition tliat they were actually 

 metamorphosed at tlie same time The study of these rocks will be re- 

 sumed in connection with the geology of the gold belt of California. 



CONDITIONS ATTENDING THE METAMORPHISM. 



In the foregoing pages the unaltered and the metamorphosed rocks of 

 the Coast Ranges have been described from a lithological point of view. 

 It remains to consider the metamorphic process as a whole in its geological 

 and chemico-physical relations. 



Proofs of jnetamorphism — Tlie divislou of opiuioii as to tlic origiu of many of 

 the crj^stalline rocks is such that it is not superfluous to insist upon the 

 proofs of the derivative character of the holocrystalline rocks and of the 

 serpentine of the Coast Ranges. It appears that at least one mineral of 

 nearly universal distribution in the granular and schistose rocks and in the 

 phthanites is especially significant in this respect, and that a sound argu- 

 ment may be based upon its occurrence independently of other evidence. 



Zoisite seems to be characteristicalh' the result of secondary processes 

 which have taken place at no very high temperature. It has never been 

 observed as an original constituent of eruptive rocks, Avhich could hardly be 

 the case if it were at all common. This merely negative evidence is sup- 

 ported by that afforded by its composition, winch includes basic hydrogen, 

 while no such compounds, so far as I know, have ever been proved to form 

 original constituents of eruptive masses, and, judging from what is known 

 of eruptions, it is difficult to conceive that they should so occur.' On the 



■ Zoisite is usually referred to the epidote grouj), of which only allauite is kuowu to occur iu erupt- 

 ive rocks. The compositiou of allauite, however, is somewhat uncertain, since, according to Professor 

 Rammelaberg, it has the oxygen ratio of garnet rather than of epidote, while there appear to.be both 

 hydrous and auhydrous varieties. According to Prof. J. 1). Dana, the hydrous allauites arc properly 

 altered forms of the species. There can be little doubt that the unaltered allauites of eruptive rocks 

 are anhydrous. 



Messrs. Fouque aud Michel-Levy regard zoisite as a scapolite, for which I know of no ground ex- 

 cepting that its centesimal composition is the same as that of meionite. The scapolites also are known 

 only as the result of secondary or metamorphic action. 

 MON XIII 9 



