chap. in. Obsidian Formation. 6^ 



of diffused matter, into the black and red specks of 

 hornblende and oxide of iron. 



Fifthly, — A compact heavy rock, not laminated, 

 with an irregular, angular, highly crystalline, fracture; 

 it abounds with distinct crystals of glassy feldspar, and 

 the crystalline feldspathic base is mottled with a black 

 mineral, which on the weathered surface is seen to be 

 aggregated into small crystals, some perfect, bat the 

 greater number imperfect. I showed this specimen to 

 an experienced geologist, and asked him what it was, 

 he answered, as I think every one else would have done, 

 that it was a primitive greenstone. The weathered 

 surface, also, of the foregoing (No. 4) banded variety, 

 strikingly resembles a worn fragment of finely laminated 

 gneiss. 



These five varieties, with many intermediate ones, 

 pass and repass into each other. As the compact 

 varieties are quite subordinate to the others, the whole 

 may be considered as laminated or striped. The laminae, 

 to sum up their characteristics, are either quite straight, 

 or slightly tortuous, or convoluted ; they are all parallel 

 to each other, and to the intercalating strata of obsidian ; 

 they are generally of extreme thinness ; they consist 

 either of an apparently homogeneous, compact rock, 

 striped with different shades of gray and brown colours^ 

 or of crystalline feldspathic layers in a more or less 

 perfect state of purity, and of different thicknesses, with 

 distinct crystals of glassy feldspar placed lengthways, or 

 of very thin layers chiefly composed of minute crystals 

 of quartz and augite, or composed of black and red 

 specks of an augitic mineral and of an oxide of iron, 

 either not crystallised or imperfectly so. After having 

 fully described the obsidian, I shall return to the subject 

 of the lamination of rocks of the trachytic series. 



The passage of the foregoing beds into the strata oi 



