Ch. xxvm.] volcanic rocks. 595 



manner and position in which they occur in the earth's crust, it will 

 be desirable to treat of their mineral composition and names. The 

 varieties most frequently spoken of are basalt and trachyte, to which 

 dolerite, greenstone, clinkstone, and others might be added ; while 

 those founded chiefly on peculiarities of texture, are porphyry, amyg- 

 daloid, lava, volcanic breccia or agglomerate, turf, scoriae, and pumice. 

 It may be stated generally, that all these are mainly composed of two 

 minerals, or families of simple minerals, felspar and hornblende ; but 

 the felspar preponderates greatly even in those rocks to which the horn- 

 blendic mineral imparts its distinctive character and prevailing color. 



The two minerals alluded to may be regarded as two groups, rather 

 than species. Felspar, for example, may be, first, common felspar 

 (often called Orthoclase), that is to say, potash-felspar, in which the 

 predominant alkali is potash (see Table, p. 608) ; or, secondly, aibite, 

 i. e. soda-felspar, where the predominant alkali is soda ; or, thirdly, 

 Oligoclase, in which there is also more soda than potash, but which 

 contains less silica than aibite ; or, fourthly, Labrador-felspar (Labra- 

 dorite), which differs not only in its iridescent hues and cleavage, but 

 also in containing less silica than aibite, and in having lime in its 

 base. Anorthite, so called from the oblique interfacial angles of its 

 rhomboidal prisms, is nearly allied in composition with Labradorite. 

 As to " glassy felspar " and " compact felspar," they cannot rank as 

 varieties of equal importance, for both the albitic and common felspar 

 appear sometimes in transparent or glassy crystals ; and compact fel- 

 spar, or petrosilex, is a compound of a less definite nature, sometimes 

 containing largely both soda and potash. It might be called a fel- 

 spathic paste, being the residuary matter after portions of the original 

 matrix have crystallized. Eecent analysis has shown that all the 

 varieties of felspar may contain both potash and soda, although in 

 some of them the potash, and in others the soda, greatly prevails. 



The hornblendic group consists principally of two varieties ; first, 

 hornblende, and, secondly, augite, which were once regarded as very 

 distinct, although now some eminent mineralogists are in doubt 

 whether they are not one and the same mineral, differing only as one 

 crystalline form of native sulphur differs from another. 



The history of the changes of opinion on this point is curious and 

 instructive. Werner first distinguished augite from hornblende ; and 

 his proposal to separate them obtained afterwards the sanction of 

 Haiiy, Mohs, and other celebrated mineralogists. It was agreed that 

 the form of the crystals of the two species were different, and their 

 structure, as shown by cleavage, that is to say, by breaking or cleaving 

 the mineral with a chisel, or a blow of the hammer, in the direction 

 in which it yields most readily. It was also found by analysis that 

 augite usually contained more lime, less alumina, and no fluoric acid ; 

 which last, though not always found in hornblende, often enters into 

 its composition in minute quantity. In addition to these characters, 

 it was remarked as a geological fact, that augite and hornblende are 



