THE DECOMPOSITION OF THE ROCKS. 77 



in fact chlorite; but that the epidote is to be attributed to the alteration of 

 these foreign particles, and not to a transformation of the feldspar sub- 

 stance, seems to me certain. The grounds for 'this view, which has not 

 hitherto been entertained, are as follows: There is no question that chlorite 

 arises from the decomposition of the bisilicates, and that it may become 

 dillused through the groundmass is equally certain. Many slides from 

 Washoe show the feldspars in a very fresh state, while the bisilicates are 

 Avholly chloritized. In such cases the chlorite cannot be due to feldspar 

 decomposition, but its diffusion through the groundmass is nevertheless 

 common. Carious feldspars appear to be impregnated with chlorite as a 

 rule when the neighboring bisilicates are undergoing chloritic decompo- 

 sition, but not otherwise; and epidote is found developing in chloritic 

 masses inclosied in feldspar when, and only when, the same process is 

 going on in chlorite patches not so inclosed, the origin of which is distinctly 

 referable to hornblende, augite, or mica. Many cases of the formation of 

 epidote have been observed in chlorite inclosed in feldspar, as convincing 

 as the instances of the transformation of chlorite arising from the bisilicates 

 which are illustrated on Plate II., though none so beautiful; but in no 

 instance in the Washoe District has epidote been seen sending its twig-like 

 crystals into feldspathic masses. 



Other alteration-products of chlorite. — Clilorite also dcgcncrates luto quartz, cal- 

 cite, and limonite, and this change is sometimes to be observed in the same 

 slide which shows its alteration to epidote. In this case, also, the dense belt 

 of chlorite which occasionally forms at the surface of a crystal of hornblende, 

 seems to offer considerable resistance to attack. An instance is illustrated 

 in Fig. 10, Plate II. Sometimes this change seems to be referable to a dis- 

 tinct period in the decomposition of the rock, as in the case shown in Fig. 

 3, Plate II., and described under slide 464. 



Character cf the chloritic mineral. TllC chlorltC arlslug frOUl tllC blsIlIcateS and 



mica shows the same optical properties, and the conversion of chlorite into 

 epidote is frequent from whichever primary mineral it may be derived. 

 There seems, however, a somewhat smaller tendency for the chlorite arising 

 from augite to change to epidote, than is displayed by that formed from 

 hornblende and mica; but the diffei'ence in this respect is not great or uniform. 



