76 LITHOLOGICAL GEOLOGY. 



in many places it is associated with anhydrite, or sulphate of lime 

 containing no water (p. 56). The borate of magnesia (boracite) and 

 polyhalite are often found in gypsum-beds ; also, rarely, hydrous 

 borate of lime (Jtayesine), as in Nova Scotia. 



4. Igneous or Eruptive Rocks. 



Igneous rocks are those which have been ejected in a melted state 

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

 most general characteristics are: (1) the' presence of a feldspar as one 

 of their constituents ; (2) the near, when not total, absence of free 

 quartz ; (3) their frequent occurrence in fissures (pp. 714, 716), as well 

 as in overlying masses, or intercalated between layers of stratified 

 rocks. Igneous rocks are not always easily distinguished from meta- 

 morphic rocks, and a few kinds of the two divisions are identical. 



In the metamorphic process, a stratified rock has sometimes been reduced to a pasty 

 state, anil in this condition has been forced into fissures, and so has taken the position, 

 and, as it cooled, the crystalline texture and aspect, of an igneous rock. Some granite 

 is an example. Again, true igneous rocks have at times resulted from the fusion (or an 

 equivalent softening) of preexisting crystalline rocks (granite, syenyte, and the like), 

 and so have derived a constitution more or less resembling that of the rock out of which 

 they were made. Thus igneous rocks, although generally containing little or no quartz, 

 may in some cases abound in grains of this mineral. 



There are two series of igneous rocks — 



1. A feldspathic series, the species containing little or no hornblende 

 or pyroxene, and hence but little iron, and of low specific gravity 

 (2-4-2-7). 



2. A hornblende-and-pyroxene series, the species containing as prom- 

 inent ingredients iron-bearing varieties of hornblende or pyroxene, 

 with often magnetite (or titaniferous iron), and hence of high specific 

 gravity (2-7-3-5). In nature, the series, however, graduate into one 

 another. 



1. Feldspathic Series. 



(1.) Gkanite (For description of granite seep. 67.) Whenever a granite presents 



in some parts a gneiss-like structure, or alternates in layers, however thick, with gneiss 

 or a related metamorphic rock, it is metamorphic granite. It may also be a metamor- 

 phic rock when no such characters exist to distinguish it. The granite of granite-veins 

 is in general a result of infiltration (called, at times, segregation), and is not of true 

 igneous origin. (See p. 721.) 



(2.) Gkanclyte (p. 68). — Consists of orthoclase in crystalline grains, with often 

 small disseminated crystals of mica, or hornblende. Color whitish, grayish, or pale 

 yellowish. G. = 2'5-2-6-i. Often graduates into porphyritic trachyte. 



A similar rock, but not properly granulyte — called sometimes white trap — consists 

 of albite or vligoch.se instead of orthoclase. (See Hunt, in Geol. Can., 1863,' p. 657.) 

 The feldspar may also be labradorite, a kind into which doleryte sometimes graduates. 



(3.) Pokphyry. — See page 71, much of the so called porphyry being a metamorphic 

 rock. Another part includes porphyritic varieties of trachyte, phonolyte, doleryte, etc. 

 Still another part is a volcanic conglomerate, in which both the pebbles and the base 



