FRAMING OF HYPOTHESES. 59 



It is well known that eruptive as well as sedimentary rocks are subject 

 to raetamorpliic action, and, since sedimentary rocks are not Infrequently of 

 nearly the same composition as eruptlv^e masses, they sliould yield analo- 

 gous results under similar circumstances. The study of metamorphics 

 sliould therefore throw light on the transformations of eruptive masses, a 

 study most intimately connected with mining geology. It will appear in 

 the sequel, as it does from the published investigations of other geologists, 

 how much caution should be exercised in deciding, from slides of altered 

 eruptive rocks, what mineral constituents are secondary. It Is certain that 

 the neglect of such caution has more than once led llthologists into grave 

 errors, and the facility with which It appears that new mineral combina- 

 tions take place, under conditions perhaps not greatly different from those 

 usually prevailing, strengthens the probability that deceptive appearances 

 are even more common than has hitherto been suspected. 



UNMETAMORPHOSED SEDIMENTARY ROCKS. 

 Macroscopical character of the rocks. ExCeptlug tllC light Cream-Colorcd ScllIstS 



of Miocene age wlilch occupy a narrow strip along the coast of California 

 from the neighborhood of Santa Cruz southward, the rocks of the qulck- 

 siher belt where unaltered are mainly sandstones of Cretaceous and Ter- 

 tiary age. (See Chapter V.) 



The sandstone of the Coast Ranges often occurs In practically unin- 

 terrupted series of beds many thousands of feet in thickness Indeed, the 

 observer can hardly fall to wonder under what mechanical conditions such 

 vast accumulations of sand can have gathered. This problem, presented in 

 many regions, though perhaps nowhere else on so large a scale, has never I 

 believe received a satisfactory solution. From the Neocomlan to the Mio- 

 cene the predominant rock of this class Is of medium grain and light color, 

 usually yellowish where exposed, bluish at some depth from the surfiice. 

 The Tejon rock, however. Is, as a rule, much lighter in color than the oth- 

 ers, and often almost white. Induration Is much more frequent among the 



and do not doubt that highly olivinitic rocks may decompose to a mass substantially composed of 

 serpentine. For the Coast Ranges of California and in part for the gold belt, however, careful study 

 has led me to very diifereut conclusions; but I do not hesitate to believe that every mineral has been 

 formed somewhere in nature by every possible method Real peridotitic serpentine occurs in the gold 

 belt and has been carefully compared with the serpentine of tie Coast Ranges. 



