ON A BASIC ROCK DERIVED FROM GRANITE. 675 



be very different for an average sample. The silica would be 

 increased by the grains of quartz, but would hardly exceed 40 to 

 50 per cent. 



The results of the analysis and of the microscopical study 

 bring into question the propriety of applying to the rock the 

 name serpentine. Modern usage seems to justify the use of the 

 designation "serpentine" for rocks composed largely of the mineral 

 serpentine. But the analysis of the green aggregate composing 

 the larger part of the rock under discussion shows a composition 

 so different from that of serpentine, that there can be no doubt 

 of the impropriety of applying this name to the rock. Analyses 

 of different portions of the green aggregate would probably 

 yield decidedly different results, so that it seems useless to 

 attempt to identify it as a whole with any mineral species ; but 

 the analysis given, as well as the optical properties, indicates 

 a much closer relationship with the chlorites than with serpen- 

 tine. This relationship is illustrated by analysis II. and III., 

 which are very similar to I., the difference being no greater than 

 would naturally result from the variability in the composition of 

 the minerals concerned. 



Cause of the alteration. — In endeavoring to ascertain the cause 

 which has led to such a complete change in the granite, it is evi- 

 dent that search must be made among the class of processes 

 to which Roth 1 gives the name of " complicated weathering." 

 For the alteration is not of a kind that could be produced by 

 the simple agents of the normal weathering of rocks, nor do the 

 facts indicate that it is a result of ordinary dynamic metamorphism, 

 though, as stated above, crushing of the rock has been an impor- 

 tant factor. The granite must have been attacked by some pow- 

 erful chemical agent, whose action was not general, but, on the 

 contrary, limited to this particular locality, and to such others as 

 show analogous alteration products. A clue to the nature 

 of the agent is afforded by the composition of the altera- 

 tion product, and by the character of the associated rocks. From 

 the analysis it is clear that the alteration has been brought about 



X J. Roth, Allgemeine und Chemische Geologie, Vol. I., p. 2. 



