484 OVERLYING ROCKS. 



hardness of the clinkstone, or compact felspar, 

 with which it is united. In such cases, where 

 the base is of a very dark colour, and the pro- 

 portion of the hornblende but small, the character 

 of the rock becomes doubtful, and it may even 

 pass into compact felspar, or rather into clink- 

 stone. In this way, very dark clinkstones are 

 frequently known by the name of basalt ; and it 

 will accordingly be found, on examining many of 

 the masses and veins in Scotland usually desig- 

 nated by this term, that they ought rather to be re- 

 ferred to the former substance. Hence the great 

 diversity in the basalts of different authors ; if 

 we even exclude those which, like hornblende 

 rock and Lydian stone, have been improperly 

 ranked under this title. 



From the presence of hornblende therefore, 

 are derived three of the most leading members 

 of the trap family, basalt, greenstone, and syenite ; 

 although it does not seem possible to give any 

 marks by which the two latter substances shall 

 always be effectually distinguished ; since, in 

 each, the basis of claystone, clinkstone, or com- 

 pact felspar, is united to crystals of hornblende, 



