Cii. XXVIIL] OF VOLCANIC EOCKS. 463 



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

 founded chiefly on peculiarities of texture, are porphjny, amygdaloid, lava, 

 volcanic breccia or agglomerate, tuff, scoriae, and pumice. It may be 

 stated generally, that all these are mainly composed of two minerals, 

 or families of simple mmerah, felspar and hornblende ; but the felspar 

 preponderates greatly even in those rocks to which the hornblendic min- 

 eral 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. 475) ; or, secondly, albite, that is to say, 

 soda-felspar, where the predominant alkali is soda instead of potash ; or, 

 thirdly, Oligoclase ; or, fourthly, Labrador-felspar (Labradorite), which 

 differs not only in its iridescent hues, but also in its angle of fracture or 

 cleavage, and its composition. We also read much of two other kinds, 

 called glassy felspar and compact felspar, which, however, cannot rank as 

 varieties of equal importance^ but both the albitic and common felspar 

 appear sometimes in transparent or cflassij crystals ; and as to compact 

 felspar, it is a compound of a less definite nature, sometimes containing 

 largely both soda and potash ; and which might be called a felspathic 

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

 have crystallized. The more recent analyses have shown that all the 

 varieties or species of felspar may contain both potash and soda, al- 

 though in some of them the one, and in others the other alkali greatly 

 prevails. 



The hornhlencUc group consists principally of two varieties ; first, horn- 

 blende, 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 in- 

 structive. 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 cr^'stals 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 mi- 

 nute quantity. In addition to these characters, it was remarked as a 

 geological fact, that augite and hornblende are very rarely associated to- 

 gether in the same rock ; and that when this happened, as in some lavas 

 of modern date, the hornblende occurs in the mass of the rock, where 

 crystallization may have taken place more slowly, while the augite merely 

 lines cavities where the crystals may have been produced rapidly. It 

 was also remarked, that in the crystalline slags of furnaces, augitic forms 



