86 LITHOLOGICAL GEOLOGY. 



(Boracite), and Poly halite, are often found in gypsum-beds ; also, 

 rarely, hydrous borate of lime {Hayesine), as in Nova Scotia. 



Gypsum is deposited from sea-water ; but the gypseous rocks 

 appear generally to have been formed by the action of sulphuric 

 acid (from decomposition of sulphurets or volcanic vapors) on 

 limestone or carbonate of lime. The action drives off the carbonic 

 acid and makes sulphate of lime, or gypsum. 



4. Igneous Rocks. 



93. Igneous rocks are those which have been ejected in a melted 

 state either from volcanoes or through fissures in the earth's crust. 

 As the crystallizing of superficial deposits may produce rocks like 

 those that are of true igneous origin, the same species in a few 

 cases occur in both divisions. Thus, there are metamorphic 



.diorite and porphyry, as well as igneous diorite and porphyry. 

 The igneous rocks differ from most of the metamorphic series in 

 the absence or very sparing occurrence of quartz. 



There are two series of igneous rocks, — the feldspathic, having 

 light colors and being of low specific gravity, and the augitic, having 

 dark colors with high specific gravity. 



1. Feldspathic series. — Consisting mainly of a feldspathic base, with, 

 often, disseminated crystals of some kind of feldspar, or of horn- 

 blende or pyroxene. Color, white, gray, bluish gray, grayish 

 brown. Gr. =3 2.4-2.8. Occasionally, as in the porphyries, dark red 

 and brown. The light colors and low specific gravity are owing 

 to the absence or sparing dissemination of iron. 



2. Augitic series. — Consisting of feldspar and hornblende or augite 

 (greenish-black or black pyroxene). Color, dark gray, dark gray- 

 ish brown, dark greenish brown, greenish-black to black. Gr. =2.9 

 -3.6. The high specific gravity and dark colors are owing to the 

 presence of iron as magnetic or titanic iron, or as a constituent 

 of the augite or hornblende. 



1. Feldspathic Series. 



94. (1.) Feldspathic Trap. — A rock consisting of crystallized feldspar, and 

 sometimes called, from its color, white trap. Varieties occurring in Canada 

 have a whitish, grayish, or pale yellowish color, also a pale fawn color. Com- 

 position of a variety from Chamhly, Canada, according to T. S. Hunt (Logan's 

 Rep. for 1853-56), consisting of orthoclase, as follows : — 





Si03. 



A1202. 



FeW 



CaO. 



NaO. 



KO. 



Ignition. 





1. Crystals. 



66.15 



19.75 







0.95 



5.19 



7.53 



0.55 = 



100.12 



2. Paste 



67.60 



18.30 



1.40 



0.45 



5.85 



5.10 



0.25 = 



98.95 



