G R A 



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G R A 



generally the predominating, as 

 mica is "the least considerable in- 

 gredient of the rock. In some 

 varieties the quartz is wanting ; in 

 others the mica; these varieties 

 have received particular names. 

 There are many varieties of granite; 

 as porphyritic granite, in which 

 large crystals of felspar occur ; 

 sienitic granite, in which horn- 

 blende supplies the place of mica ; 

 chloritic, or talcy granite, composed 

 of quartz, felspar, and talc or 

 chlorite, instead of mica ; felspathic 

 granite, &c. &c. Granite is a 

 compound plutonic or igneous 

 rock, unstratified and crystalline, 

 of a granular structure, whence its 

 name. Prom its great relative 

 depth, granite is rarely met with 

 but in mountainous situations, 

 where it appears to have been 

 forced through the more superficial 

 covering. "It was at one time 

 supposed that granite was peculiar 

 to the lowest portions of the rocks 

 composing the crust of the earth, 

 and that, in fact, it constituted the 

 fundamental rock upon which all 

 others had been formed, and was not 

 discovered higher in the series. This 

 opinion has given way before facts, 

 for we find granitic rocks in situ- 

 ations where they must have been 

 ejected subsequently to the period 

 during which the cretaceous group 

 was deposited, as also in other 

 places, into which they must have 

 been thrust at intermediate periods 

 down to the oldest rocks inclusive." 

 Granite is said to contain forty-eight 

 per cent, of oxygen. Granite being 

 an igneous rock, no organic fossil 

 remains could be expected to be 

 found therein, nor have any 

 ever been discovered ; nevertheless 

 granite is occasionally found over- 

 lying strata containing fossil or- 

 ganic remains, as in Norway; a 

 mass of granite has been discovered 

 superincumbent on secondary lime- 

 stone, which contains orthocerata, 



&c. From these circumstances 

 there can no longer exist a doubt 

 but that granite has been formed 

 at different periods, and is of 

 various ages. A comparatively 

 modern granite may be observed in 

 the Alps penetrating secondary 

 strata, such secondary strata con- 

 taining fossils, such as belemnites, 

 referrible to the age of the English 

 lias. Although granite has often 

 pierced through other strata, it has 

 seldom, or ever, been found to rest 

 upon them, as if it had overflowed. 

 Granite almost invariably consti- 

 tutes the central ridges of mountain 

 chains, occupying the highest and 

 the lowest position in their strati- 

 fication. That it has been forced 

 up by some violent convulsions of 

 nature, which have shaken our 

 planet to its very centre, is beyond 

 doubt. The indented ridges, the 

 ragged precipices, the bristling 

 peaks, by which these chains are 

 characterised, prove to demonstra- 

 tion the violence exerted in their 

 production. In this respect they 

 present a marked contrast to those 

 more convex mountains, and undu- 

 lating ridges of hills, whose mass 

 was quietly deposited by the last 

 retiring sea, and has subsequently 

 remained undisturbed by any violent 

 revolution. Felspar is by far the 

 largest constituent of granite, and 

 in some kinds it is found in large 

 whitish crystals of irregular forms, 

 occasionally of one or two inches in 

 length. Granite of this kind, how- 

 ever beautiful it may be to the 

 eye, is not well adapted for 

 buildings, the felspar being subject 

 to decomposition from the continued 

 action of the atmosphere. Water- 

 loo-bridge is unfortunately built of 

 this perishable kind of granite. It 

 may be considered as a general law, 

 that wherever granite rises to any 

 height above the surface of the 

 earth, the strata of other surround- 

 ing rocks rise towards it. The 



