174 



GRANITIC ROCKS. 



these modified limestones, even the shells of tnollusks or madrepores, are 

 fr.und changed into magnesia ; this clearly proves that an action subsequent 

 to the formation of the deposit has produced dolomisa'tion, for no shell or 

 madrepore exists which naturally contains magnesia, either in the living or 

 fossil state, where the deposit has undergone no modification. 



Feldspathic porphyries are often so characterized that there can be no 

 doubt of their igneous origin. Not only are they found in veins in the 

 midst of rocks, but they act like trachytes, in passing through split 

 rocks, the fragments of which they glue together to form conglomerates ; 

 they often unite themselves in the most intimate manner to arena'ceous de- 

 posits which harden in their vicinity. 



19. Granitic rock*. There can be no doubt as to the igneous 

 nature of the preceding rocks, from the manner in which they are 

 injected into all kinds of deposits, and from the modifications they 

 produce in the substances they pass through or upheave. The 

 same is true of all granitic rocks, that is of granite properly so 

 called, of syenites, which resemble them more or less in appear- 

 ance, and pass into them in all manners, of certain gneiss rocks, 

 which belong immediately to one or the other, &c. In short, it is 

 inferred from a great mass of observations, collected first in Eng- 

 land by Dr. Macculloch, afterwards verified by other geologists, 

 that the granites, which are massive rocks, and therefore distinct 

 from aqueous deposits, which are ordinarily stratified, act, on their 

 appearance, exactly like the traps, diorites, and porphyries. 



20. In the valley of Glen-Tilt, in Scotland, granite is found 

 injected into calcareous deposits, which alternate with argilla'ceous 

 schists (Jig. 284), into which it sometimes forces separate masses 

 (a); fragments of limestone (6) are also found enveloped in the 

 granite itself. In other places vertical veins traverse the rock 

 (Jig. 285), sometimes entirely, sometimes terminating in pointed 



Fig. 284. Fig. 285. 



Injection of granite into different rocks. 



19. What is the origin of granitic rocks? What rocks are included 

 under the name of granitic rocks ? 

 2C. What circumstances prove the igneous origin of grani'ic rocks? 



