Detrital Theory of Geology. 303 



In examining the rocks formed from granite (including 

 their organic remains), there are none which require a more 

 careful consideration than the gneiss system, the same 

 being first in order after granite, but in chemical composi- 

 tion containing an excess of lime as compared with that 

 rock. In this system hornblende schist occupies a pro- 

 minent place, in conjunction with talc, chlorite, and mica 

 schists, amongst which latter magnesia takes a more pro- 

 minent place than lime. 



The texture and appearance of the metamorphic rocks 

 closely resemble that of granite, and the extreme com- 

 minution of their crystals closely approximates the primary 

 plutonic rocks. They appear to consist of little else than 

 crystals of granite, worn and comminuted, and thrown 

 together into strata, or thinly laminated beds of granite 

 debris deposited from water, which had worn the granite 

 rock in one part of the ocean, and by currents conveyed it 

 to another. 



By observing Table I., the per-centage of lime and mag- 

 nesia in hornblende exceeds that in mica or felspar very 

 considerably ; and hornblende is a rock very generally dis- 

 tributed in the granite series, though more superficial and 

 more limited in quantity than either felspar, quartz, or mica. 

 Hence hornblende is naturally looked upon as the source 

 of lime and magnesia in the metamorphic rocks, if these 

 rocks are formed MERELY by the disintegration of the 

 granite. But in the gneiss system hornblende crystals 

 occur in such great abundance as hornblende schists, that it 

 must be inferred that this schist, in the gneiss, represents 

 fully and ENTIRELY all the hornblende derivable from the 

 disintegration of granite. 



Now, in the gneiss system, dolomite (composed of equal 

 proportions of lime and magnesia), is occasionally found, and 

 the talc schist in the same system contains as much as 



