Geology. 365. 



views of Sir James Hall, Bart, respecting the revolutions on the earth's sur- 

 face. — See Edin. Transactions, vol. vii. p. 169. 



44. Mr Poulett Scrope's arrangement of Volcanic Rocks. — As we be- 

 lieve there are few of the geologists of the present clay who have studied 

 the volcanic phenomena, and examined the volcanic districts with so much- 

 attention as Mr Poulett Scrope, we shall lay before our readers his new 

 arrangement of volcanic rocks, of which he has given an account in a 

 late Number of the Quarterly Journal. 



GENUS I. TRACHYTE. 



Species A, Compound trachyte, with mica, hornblende, or augite, sometimes 

 both, and grains of titaniferous iron. 



B, Simple trachyte, without any visible ingredient but felspar. 



C, Quartziferous trachyte, when containing numerous crystals of 

 quartz. 



D, Siliceous trachyte, when there appears to have been introduced 

 a great deal of silex into its composition. 



) 



GENUS II.— GRAYSTONE. 



Species A, Common gray stone, consisting of felspar, augite, or hornblende 

 and iron. 



B, Leucitic gray stone, when leucite supplants the felspar. 



C, Melilitic gray stone, when melilite is substituted for felspar, &c. 



GENUS III. BASALT. > 



Species A, Common basalt, composed of felspar, augite, ^and iron. 



B, Leucitic basalt, when leucite replaces the felspar. 



C, Basalt with olivine, in place of felspar. 



D, Basalt with Hauyne, in place of felspar. 



E, Ferruginous basalt, when iron is the predominant ingredient. 



F, Augite basalt, when pyroxene or hornblende compose nearly the 

 whole of the rock. 



The colour of the rocks of the graystone genus is universally of some 

 tint of gray, generally lead-gray, greenish iron, purplish, or slate-gray, with 

 the exception only of the vitrified varieties, some of which have assumed a 

 black colour, which, however, passes away under the blowpipe, and is suc- 

 ceeded by the usual gray tint. 



In general the colour of the mass is deeper in proportion to the quan- 

 tity of augite matter in its composition, the felspar being always of a light 

 colour, the augite of a darkish green or black hue, and the iron of a dark- 

 brown or black. 



The proportion of felspar, or its substitutes in trachyte, may be reck- 

 oned at or above 90 per cent, the remainder being composed of augite, or 

 the ferruginous minerals. 



In graystone, felspar or its substitutes composes more than 75 per cent. 

 "When these minerals are in less proportion than 75 per cent, the rock 

 should be classed as basalt. 



Another auxiliary test is the specific gravity of the substance reduced 



