kinds or ROCKS. 87 



Certain dikes in the Montreal Mountain are of this kind, except that in 

 some the feldspar is a soda feldspar. Found also at Lachine. A similar 

 rock occurs in Australia in Prospect Hill, near Paramatta, New South Wales. 



(2.) Porphyry. — A rock the mass or hase of which is a compact uncleavable 

 feldspar, containing disseminated crystals of feldspar (orthoclase or oligoclase), 

 giving it an appearance, when polished, of being spotted with white or some 

 pale color, the spots rectangular or nearly so in form. Color, base grayish, 

 purplish, to deep red and brown ; and crystals either large or minute. The feld- 

 spar either orthoclase or oligoclase ; mica or hornblende sometimes present. 

 Specific gravity of the red antique porphyry, 2.62-2.77. 



This rock closely resembles true metamorphic porphyry, like that from the 

 vicinity of Boston. The antique green porphyry of Greece is diabase por- 

 phyry (g 85). G. = over 2.9. 



Much of the so-called porphyry of the Andes and Mexico is a porphyry con- 

 glomerate, in which both the pebbles and base are spotted with feldspar crys- 

 tals, and the texture looks homogeneous until closely examined. 



(3.) Phonolite (Clinkstone). — Compact, of grayish blue and other shades of 

 color, more or less schistose or slaty in structure; tough, and clinking under the 

 hammer like metal when struck, whence the name. Often contains dissemi- 

 nated crystals of glassy feldspar and hornblende, and sometimes mica. Con- 

 sists of glassy feldspar (orthoclase or oligoclase), with nepheline or a zeolite 

 '(g 68). Action of muriatic acid separates it into a soluble and an insoluble state, 

 the former including all the nepheline or zeolite. Analysis by Jenzsch of the 

 Bohemian phonolite, — Silica, 56.28, alumina, 20.58, lime, 0.46, soda, 9.07, potash, 

 5.84, lithia, 0.05, protoxyd of iron, 2.86, protoxyd of manganese, 1.45, magne- 

 sia, 0.32, titanic acid, 1.44, phosphoric acid, 0.29, loss by ignition, 1.29, chlorine, 

 0.54, sulphur, 0.02 ; from which he deduces that it consists of 53.55 of glassy 

 feldspar, 31.76 of nepheline, with some hornblende and sphene (nepheline or 

 zeolite). Other phonolites consist of 18 to 50 per cent, of soluble silicate. 



Common Phonolite. — Schistose, without distinct feldspar crystals, and contain- 

 ing a zeolite disseminated through the mass, which is usually regarded as an 

 essential ingredient of the rock. 



A peculiar feldspathic igneous rock occurring at Lachine, in Canada, having 

 a reddish, fawn-colored base, brittle, G-. = 2.414, and H. = 5, consists, like pho- 

 nolite, of feldspar (orthoclase) and a zeolite (probably natrolite) in nearly equal 

 parts. It forms dikes, and is in the same region with the dikes of white trap 

 mentioned in § 94 (1). 



Porphyritic Phonolite. — Containing disseminated glassy feldspar crystals. 



Phonolitie Lava. — Differing from trachyte in being more cellular and in not 

 having the peculiar roughness of trachyte over a surface of fracture. 



(4.) Trachyte. — Color, pale grayish blue, rarely greenish, yellowish, red- 

 dish ; texture peculiarly rough to the feel, and usually porous. Often contains 

 disseminated crystals of glassy feldspar and hornblende, and some little free 

 quartz, also mica. G-. = 2.6-2.7. Decomposed by the action of muriatic acid 

 into a soluble and an insoluble silicate, the former in less proportion than in 

 clinkstone, or 10 to 14 per cent. Composition of the whole (from Drachenfels), 

 according to Abich, — Silica, 67.09, alumina, 15.64, potash, 3.47, soda, 5.08, 

 2ime, 2.25, oxyds of iron, 4.59, magnesia, 0.98, protoxyd of manganese, 0.15, 



