1 78 On the Granite formation 



Chemical analysis of both minerals which I have made, 

 also shew a considerable difference between them; but I 

 have not yet had time to make an analysis sufficiently cor- 

 rect for publication. 



Graduation. 



In the huge granite masses of this district, granite may be 

 seen graduating into all the varieties of hornblende rock, 

 hornblende slate, gneiss, friable gneiss, sometimes micace- 

 ous granite; but these changes are most distinctly visible 

 about the junction between granite ranges and the schistous 

 series. 



Specimens can be procured shewing a most perfect gra- 

 duation between the granite and hornblende rock, (black 

 granite,)* and as this rock is found in the midst of granite 

 formations, exhibiting all the characteristics of granite, it 

 cannot be well doubted that black granite is but a mode 

 of hornblendic granite, and that the term trap, sometimes ap- 

 plied to it, is improper, if by it is meant a rock intruded 

 posterior to the granite. 



I have above remarked, that the granite graduates into 

 gneiss, and as portions of gneiss retaining their fissile 

 cleavage are found embedded in the granite, it is plain that 

 the change from gneiss into granite cannot have been pro- 

 duced by the action of heat, as seems to be the opinion of 

 some writers. 



Dr. Benza (Madras Journal, vol. iv. page 285) has re- 

 marked, that granite does not graduate into basalt, in which, 

 as far as my observation goes, he is certainly quite correct; 

 no direct graduation between granite and basalt ever taking 

 place. 



* The graduation here alluded to, may be called a " Mineralogical 

 graduation." A " Geological graduation" may have place in the gra- 

 dual change between two masses or formations on a line which joins 

 the two; such however I have never seen anywhere, nor do I recollect 

 a perfect description of such a change. 



