450 Scientific Geology. 



examination of specimen No. 1222 with a microscope, has led me 

 to the belief that it contains a minute quantity of native gold. 

 The quantity is so very small that an assay cannot be made of 

 it : yet it certainly bears an exact resemblance to native gold. The 

 specimen is an extremely hard variety of compact feldspar approach- 

 ing flinty slate, from the Blue Hills. I should not allude to this cir- 

 cumstance were not the porphyry of South America rich in this 

 metal. 



Theoretical Considerations. 



Porphyry,or rather its base, has more the appearance of having once 

 been melted than any of the unstratified rocks, except perhaps some of 

 the vesicular traps. It has been thought by some distinguished wri- 

 ters that compact feldspar ought to be regarded as a mineral species 

 distinct from common feldspar, chiefly because soda is found in it 

 more frequently than potassa, and often exclusively. But one has only 

 to examine the analyses of these two substances, to see that in this re- 

 spect there is great diversity among both varieties. If compact feld- 

 spar is common feldspar or albite that has been melted in connec- 

 tion with other minerals, we ought to expect, as I have remarked in 

 another place, that its composition would not coincide with that of com- 

 mon feldspar. And that it does result from this change in common 

 feldspar, I can hardly doubt, when I often see specimens that have not 

 entirely lost their foliated structure, being intermediate between the 

 two minerals. And then the chemical effects that have been produc- 

 ed on other rocks in the vicinity 'of compact feldspar, (examples of 

 which have been mentioned under greenstone,) clearly point us to an 

 igneous agency. 



I have elsewhere elucidated the argument in favour of the igneous 

 origin of all rocks that are porphyritic, drawn from the chemistry of 

 the subject. If that argument be valid, it is obvious that it will apply 

 with peculiar force to the rock under consideration. 



It is not uncommon to meet with specimens of porphyry that exhib- 

 it traces of an originally slaty structure in all or apart of the materials 

 composing it. This clearly points us to a slaty rock as the source 

 from which porphyry was derived. And sometimes fragments of this 

 rock, along with fragments of compact feldspar, flinty slate, &c. are 

 scattered through the mass as if partly melted down ; very much as 

 fragments appear in the slag of a furnace. They seem to be all but in- 

 corporated with the paste, and the whole mass presents an appearance 



