OVERLYING HOCKS. 485 



while, in both, a similar structure prevails. The 

 present distinctions, as already observed, seem to 

 be regulated chiefly by colour; those rocks in 

 which the three substances above-mentioned are 

 of a pale whitish, yellowish, or reddish hue, 

 being ranked as syenite ; and those where they 

 are grey, greenish, or generally of a dark colour, 

 as greenstone. A better distinction appears to 

 consist in the predominance, or otherwise, of 

 those substances ; the rocks in which they exceed, 

 being classed with the former, and those in which 

 the hornblende is equally intermixed, or predo- 

 minant, with the latter. By duly considering 

 these characters, it will generally be easy to des- 

 cribe an individual rock without much hazard of 

 misapprehension ; although cases will often oc- 

 cur, where it will be impossible to determine to 

 which of the two the substance in question ought 

 to be referred. It may be added to this account 

 of hornblende as an ingredient of these rocks, 

 that it is frequently found in the older porphy- 

 ries ; occurring, in these, generally in the shape of 

 imbedded crystals, but sometimes so abundantly 



