GEOLOGICAL MEMOIRS, 



cc. With deposition of scales of mica, talc-, or clay-slate. 

 (See II. Class. Group of Stilpnolites.) 



3. Insoluble or unaltered in hydrochloric or sulphuric acid. 

 (Silicates.) 



a. Silicate of magnesia with oxydulous iron, lime, &c. 

 Dark-coloured, garlic-green to black-green, and black. 

 \j a. Group. Silicate of magnesia (magnesite) . Amphi- 

 bolite, pyroxenite, and talc-slated] 



b. Strong silicates, crystalline or amorphous ; hardness=7 

 or 2-3. Soluble in fluoric acid. Variously coloured. 

 [8. Group. Silicates. Quart zite, hornstone, lydian- 

 stone, flint, and opal.] 



c. Silicate of alumina with potash, soda, &c. Natural 

 slags and glasses. Grey, pitch-brown, or black. [9. 

 Group. Hyalolites. Pitchstone, perlite, obsidian, and 

 pumice.] 



II. Class. MIXED CRYSTALLINE EOCKS. 



I. Order. (Alabradorite.) Rocks without labradorite, never con- 

 taining augite, but generally quartz. 



Essential constituents : Orthoclase, talc, albite, sanidine, quartz, 

 mica, chlorite, hornblende ; or, instead, smaragdite and garnet. 

 Accessory constituents : Quartz, mica, garnet, tourmaline, topaz, 

 zircon, pistazite, iron-pyrites, magnetic pyrites, titanite; seldom 

 magnetic iron or zeolites. Never present : Pyroxene, diallage, and 

 hypersthene. Mixture : generally distinct. 



I. Suborder. Partially soluble in hydrochloric acid. 



1. "Without effervescence; containing 12-30 per cent, of decom- 

 posable and 70-88 per cent, of undecomposable parts. Chief con- 

 stituents : sanidine and potash-albite indistinctly mixed, often with 

 prominent sanidine-crystals, and then porphyritic. Accessory con- 

 stituents : Quartz, magnetic iron, titanite, and zeolite. Colour : 

 white-grey, greenish-grey, dark-grey, or reddish-brown, and then 

 resembling felsite. Sp. gr. = 2*5 to 2*68. Texture : dense, granular, 

 or porphyritic, generally rough and porous. Weathering : leather- 

 yellow, white, and clayey. [1. Group. Sanidite. Trachyte and 

 domite, trachytic porphyry, perlite in part, phonolite, and ande- 

 site.] 



2. With effervescence ; containing 20-80 per cent, of soluble 

 parts (carb. lime). The residue consists of mica- and talc-scales. 

 (See Group 3. Talc-mica-slate, &c.) 



II. Suborder. Not decomposable in hydrochloric or sulphuric acids, 

 and giving no water on heating to redness. Never containing 

 sanidine, and perhaps no zeolites. 



