Prof. Beck on Igneous Action, as exhibited in Neio York. 341 



rocks, to be of any, that is, of all ages ; some of that which is 

 visible in the crust of the globe may have been solidified from 

 fusion before the production of any of the strata ; other granite 

 has been melted or remelted at various later periods ; granite may 

 yet be forming in the deeper parts of the earth, round the centres 

 of volcanic fires ; but in general we must look on this rock as 

 characteristic of particular circumstances accompanying igneous 

 action, not as belonging to particular periods of geological his- 

 tory."* 



Granite of the ordinary kind is composed of quartz, feldspar and 

 mica, and it is somewhat remarkable, that although there may be 

 considerable variations in the proportions of these substances, they 

 would give rise to only slight differences in chemical composition. 

 These constituents, according to De la Beche and Phillips, are 

 Silica, .... from 7300 to 75-00 



Alumina, .... " 1090 " 1383 



Potash, .... " 7-48 " 9-80 



together with small proportions of other bodies, as magnesia, 

 lime, oxide of iron, oxide of manganese, and fluoric acid. 



Moreover, there does not appear to be a very remarkable dif- 

 ference in chemical composition between common granite and 

 those rocks which are confessedly of igneous origin, except per- 

 haps that arising from the fact that in the latter, the mica is gen- 

 erally less abundant, and the quantity of hornblende is greatly 

 increased. 



If now it should be asked, whether these igneous products 

 owe their origin to the fusion of ordinary granite, were we to 

 attend exclusively to the composition of the rocks, the answer 

 would probably be an affirmative one. Such indeed seems to be 

 that implied in the statements of De la Beche, Phillips, and other 

 geologists.f But if we look to the imbedded minerals, it will be 

 found extremely difficult to reconcile their chemical composition 

 and mode of formation with this view. Although it may be 

 freely admitted that the different varieties of granite, when sub- 

 jected to intense heat, might produce rocks not unlike those 



* Treatise on Geology, I, 108—111. 



t It is qualified, however, by the admitted difference between the granitic and 

 trappean rocks, the former being more prevalent at the earliest periods, — a differ- 

 ence which is ascribed to a " modification in the condition of things." 



