92 



LYELL'S ELEMENTS OF GEOLOGY. 



Volcanic Rocks. 



materials round the orifice of eruption, gives rise to a conical 

 mound, in which the successive envelopes of sand and scorice 

 form layers, dipping on all sides from a central axis. In the 

 mean time a hollow, called a crater, has been kept open in the 

 middle of the mound by the continued passage upwards of steam 

 and other gaseous fluids. The lava sometimes flows over the 

 edge of the crater, and thus thickens and strengthens the sides 

 of the cone ; but sometimes it breaks it down on one side, and 

 often it flows out from a fissure at the base of the hill. (See 

 Fig. 86.) 



I have given a full history and description of the phenomena 

 of recent volcanos in the Principles of Geology, and cannot re- 

 peat them here, but shall merely consider the characters of the 

 igneous rocks as they appear to a geologist in the earth's crust. 

 The subject may be treated of in the following order ; first, the 

 mineral composition, internal -texture, and nomenclature of vol- 

 canic rocks ; secondly, the manner and position in which they 

 occur in the earth's crust, and their external forms ; and, lastly, 

 the connexion between the products of modern volcanos and the 

 rocks usually styled trappean. 



Mineral composition and texture^ — First, in regard to the 

 composition of volcanic rocks, the varieties most frequently 

 spoken of, are basalt, greenstone, syenitic greenstone, clink- 

 stone, clay stone, and trachyte ; while those founded chiefly on 

 pecuHarities of texture, are porphyry, amygdaloid, lava, tufl", 

 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, some almost entirely of 

 hornblende, others of felspar. 



These two minerals may be regarded as two groups, rather 

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

 felspar, that is to say, potash-felspar, in which the alkali is pot- 

 ash (see Table, p. 102.); or, secondly, albite, that is to say, 

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

 thirdly, Labrador-felspar (Labradorite), which differs not only 

 in its iridescent hues, but also in its angle of fracture or cleav- 

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

 kinds, called glassy felspar and compact felspar, which, how- 

 ever, cannot ranJjL as varieties of equal importance, for both the 

 albitic and common felspar appear sometimes in transparent or 

 glassy crystals ; and as to compact felspar, it is probably a 

 compound of a less definite nature, sometimes containing, ac- 

 cording to Dr. MacCulloch, both soda and potash. 



The other group, or hornblende, consists principally of two 

 varieties ; first, hornblende, and, secondly, augite, which were 



