Descriptive Arrangement of Volcanic Rocks. 33 



state, under which chcumstances the hghtest colored felspar 

 rocks sometimes assume a blackish hue.* 



Speaking generally, the colour of the mass is deeper in 

 proportion to the quantity of augitic matter in its composi- 

 tion, the felspar being always of a light colour, the augite a 

 darkish green or black, and the iron a dark-brown or black. 

 The proportion of felspar, or its substitutes, which exists in 

 trachyte, may be reckoned at, or above, ninety per cent., the 

 remainder being composed of augite, or the ferruginous min- 

 erals. In greystone, felspar or its substitutes composes more 

 than seventy-five per cent. ; when these minerals are in less 

 proportion than seventy-five per cent., the rock should be 

 classed as basalt. 



Another auxiliary test, in which greater confidence may 

 be reposed, is the specific gravity of the substance when re- 

 duced to powder. In fact, the specific gravity of the augitic 

 and ferruginous minerals is so greatly superior to that of fel- 

 spar, that an observation of this nature will indicate the gen- 

 eral proportion of these two classes of minerals in any vol- 

 canic rock. In general, the specific gravity of trachyte will 

 be found not to exceed 2.7, that of greystone 3.0, while ba- 

 salt occasionally reaches 3.50, which is much above the spe- 

 cific gravity of augite alone, and caused by the presence of 

 a quantity of iron in a metallic state. 



A third test consists in the colour of the glass, produced 

 by fusion of the mineral before the blowpipe. That resulting 

 from trachyte is light coloured, and nearly transparent. The 

 glass of greystone is darker, and spotted with numerous 

 green or black specks, often of a green colour, bearing a 

 constant ratio to the proportion of ferruginous minerals in 

 the rock. Basalt melts into a dark green, or black enamel. 

 Observations which have often been made on these rocks, 

 and which, without being insisted on as infallible criteria, 

 will yet often assist in distinguishing them, are — 



1. That leucite has not been found to occur in any tra- 

 chyte, only makmg its appearance when the proportion of 

 the heavier minerals is considerable ; rarely in greystone, 

 oftener in basalt. 



* Apparently derived from the bitumen, which appears, from chemical anal- 

 ysis, to be present in this condition of the rock, and which is volatilized on ex- 

 posure to the blowpipe. 



Vol. XV.— No, 1. 5 



