THE DECOMPOSITION OF THE KOCKS. 75 



exact representations of such cases. The chlorite, with only one or two ex- 

 ceptions, has the same characters described elsewhere.' The Washoe chlorite 

 evinces a considerable solubility, which can be traced in many series of slides, 

 notably in those from the McKihhen Tumu'I. In the early stages the pseudo- 

 morphs after hornblende are vevy tine; later the form of the hornblendes is 

 obscured, and irregular patches of chlorite appear in the groundmass; at 

 last this mineral appears diffused through the rock, settling in bands round 

 a])atites, magnetites, or other solid minerals, and penetrating partially decom- 

 posed feldspars. Frequently too it occupies microscopic veins traversing the 

 slide. One such observation would perhaps be open to great question, but 

 scores of similar cases occur in the numerous slides examined. 



Augite and mica. — Botli auo-itc aud mica exhibit the same tendency to pass 

 into chlorite as hornblende, and neither the process nor the result commonly 

 differs in any way from that just described A preliminary change of augite 

 to uralite is, however, not uncommon in the diabases, and in a single slide of 

 augite-andesite the same alteration appears, though several other thin sections 

 from the same cropping show nothing of it. On the whole augite seems to 

 be somewhat more disposed to decomposition than hornblende, and cases are 

 numerous in which the hornblendes of a rock retain their freshness, when 

 the augites are completely altered; but in some instances the augites 

 have resisted longer than the hornblendes. Mica, on the other hand, cer- 

 tainl}^ yields somewhat less readily than the bisilicates, to which it is so 

 closely allied, though it is often wholly changed to chlorite. 



Formation of pyrite. — lu vcry uumerous cases pyrite has been observed in rela- 

 tions indicating that it is formed directly from hornblende and augite, appar- 

 ently at the same time as chlorite. The two products do not seem to me 

 dependent upon one another, for chlorite occurs where no pyrite is found, 

 and the process of conversion to chlorite is not visibly modified by the sinnd- 

 taneous growth of pyrite. The indications are, therefore, that the two pro- 

 cesses are wholly independent of and not inconsistent with each other. 



Epidote formed from chlorite. — Epidote is usually cousidcrcd as a direct result 

 of the decomposition of the bisilicates, but in Washoe such a transforma- 

 tion, if it occurs, must be exceptional, for it was not recognized in a single 



■See pp. 84,211, etc. 



