80 LITHOLOGICAL GEOLOGY. 



b. Pctrosilex. 



(1.) Orthoclase-Petrosilex. — Color, whitish, greenish; lustre somewhat 

 waxy, dull; specific gravity, 2.6-2.7. A greenish-gray specimen from Brittany 

 afforded Berthier — Silica, 75.4, alumina, 15.5, potash, 3.8, magnesia, 1.4, oxyd of 

 iron, 1.2, — whence it consists of orthoclase, feldspar, and some quartz. 



(2.) Albite-Petrosilex. — Similar to the preceding. A variety from Orford, 

 Canada, afforded T. S. Hunt (Logan's Report for 1853-56)— Silica, 78.55, 

 alumina, 11.81, soda, 4.42, potash, 1.93, lime, 0.84, magnesia, 0.77, protoxyd of 

 iron, 0.72, loss by ignition, 0.90 = 99.94. This rock graduates into feldspar- 

 euphotide. 



(3.) Diorite-Petrosilex. — Compact diorite, and consisting, therefore, of 

 albite and hornblende. Color, grayish white, greenish white. Occurs in Orford, 

 Canada (T. S. Hunt, Logan's Report, 1853-56). 



(4.) Garnet-Petrosilex. — A pure, compact, garnet rock, of a whitish color, 

 with spots of disseminated serpentine. Specific gravity, 3.3-3.5. Composition of 

 the base of this rock, from Orford, Canada, according to T. S. Hunt — Silica, 38.60, 

 alumina, 22.71, lime, 34.83, magnesia, 0.49, oxyd of iron and manganese, 1.60, 

 soda (with a trace of potash), 0.47, loss by ignition, 1.10 = 99.80. Specific gravity, 

 3.522-3.536. It is exceedingly hard and tough. Graduates into garnet- 

 euphotide. A similar rock occurs at St. Francois, Canada. 



c. Exiplwtides. 



(1.) Feldspar-Euphotide. — Tough compact, light-green or grayish, con- 

 sisting of a minutely-granular feldspathic base with disseminated diallage or 

 smaragdite. 



(2.) Epidote-Euphotide. — Similar to the preceding, but more tough, and 

 heavy. Specific gravity, 3.1-3.4. The base a compact whitish epidote (called 

 hitherto saussureite), according to T. S. Hunt. From the Alps. 



(3.) Eclogite, or Garnet-Euphotide. — Either whitish, greenish, or reddish; 

 very tough and heavy. Specific gravity, 3.2-3.5. The eclogite of Europe con- 

 tains grass-green smaragdite in a reddish garnet base. The related rock from 

 Canada, according to T. S. Hunt (Logan's Report for 1853-56, p. 450), contains 

 grayish cleavable hornblende or pyroxene in a whitish or yellowish base. Part 

 of the base is in some cases feldspathic : its low specific gravity — 2.8 — serves to 

 distinguish this variety. 



4. Hydrous Magnesian Series. 

 86. The hydrous magnesian series, characterized by the presence 

 of the hydrous magnesian minerals talc, serpentine, or chlorite ($ 66), 

 ranges from a granite-like rock called protogine (containing the con- 

 stituents of granite, excepting talc in place of mica) down to the 

 semicrystalline talcose and chloritic slates ; and also to compact 

 flinty rocks near aphanite. Besides these, there are the serpentine 

 rocks. Talc and serpentine are silicates of magnesia and water 

 alone, while chlorite contains alumina and oxyd of iron. The 

 chloritic rocks consequently abound often in hornblende, and are 

 frequently associated with rocks of the hornblende series. The 



