174 On the Granite formation 



Basalt Veins. 



Veins and dykes of basalt, and also masses of basalt dif- 

 fused into the mass, occur in the granite of this district, 

 and in other localities in south India. This phenomena is a 

 point of some consequence in Petrology. Basalt dykes in 

 granite appear to be very uncommon in Europe, indeed it 

 may be doubted if they have ever been seen. For although 

 mentioned by Thomson, Mineralogy vol. ii. p. 222, yet 

 Bakewell and Phillips (Encyclopedia Metropolitana, Geology, 

 page '752,) contradict him, both calling the rock gneiss ; and 

 on collating the remarks of Phillips and Bakewell, it will be 

 seen that they contradict each other ; the one asserting that 

 the gneiss is changed to hornstone, the other that the basalt 

 is so changed. 



The rocks to which the term basalt has been applied by 

 different writers are so various, that I must enter on the 

 subject by defining my own application of the term, which 

 may be very well inserted here, as basalt in south India is 

 found in granite masses principally. 



McCulloch* proposes to confine the use of the term to 

 a minute aggregation of hornblende ; but if this is done, I 

 shall be left without a name for the rock in question. 



Black granite (hornblende rock) is an aggregation of 

 hornblende and quartz or felspar, with the hornblende in 

 such large proportion as to give to the rock a black colour, 

 and to render the grains of quartz and felspar invisible, un- 

 less examined closely. 



When the structure is so minute as to be almost invisible 

 without a lens, and when its fracture is semi-conchoidal, it 

 will be best termed basaltic hornblende. 



A minute aggregation of hornblende alone, which shews 

 no glistering grains of quartz or felspar, will be best termed 



* Geology, vol. ii. page 103. 



