ON A BASIC ROCK DERIVED FROM GRANITE. 673 



with, or replacing, the green scales, colorless scales with similar 

 cleavage, but no pleochroism, and having strong double refrac- 

 tion. In this case the mineral is probably muscovite. It is 

 much less abundant than the chloritic mineral and disappears as 

 the alteration becomes more complete. By this it is not meant 

 to imply that the muscovite is an essential step in the process of 

 change, as in most cases there is no trace of it, even in the earlier 

 stages of alteration. Other substances are present in minor 

 quantity, and generally of undeterminable character. In many 

 cases they are evidently the result of the alteration of the normal 

 green aggregate by ordinary surface agents, with the production 

 of iron oxide, carbonates, etc. Such weathering often brings 

 out very clearly a wavelike banding in the 'sections, which very 

 closely resembles flow structure. When this structure appears 

 in a section of the most highly altered rock, composed of the 

 very low, doubly refracting aggregate, the likeness to a section of 

 a glassy volcanic rock is striking. 



Cataclastic structure is very pronounced in most sections, 

 when the alteration has not proceeded so far as to hide it, and 

 there can be no doubt that the crushing played an important 

 part in the process of change. 



Chemistry of the process. — As microscopical study gives no defi- 

 nite information in regard to the chemical composition of the 

 altered granite, an analysis has been made of a carefully selected 

 sample. For this purpose a specimen was chosen representing 

 the extreme result of the process of alteration, being nearly free 

 from quartz, with a deep green color and waxy lustre. A thin 

 section cut from the specimen shows a mingling of green and 

 yellow aggregates, with no trace of feldspar or quartz. The 

 results of the analysis are shown in column I. No analysis has 

 been made of the fresher granite, because even the best speci- 

 mens are so much altered that the results obtained would 

 give no clearer idea of the original composition of the rock than 

 can be gathered from a consideration of its mineralogical com- 

 position. 



