Prof. Beck on Igneous Action, as exhibited in New York. 341 
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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- 
9% : 
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, F : é i from 73:00 to 75:00 
Alumina, . ‘ é ; * 10:90 “ 13°83 
Potash, : ‘ ‘ SURES SPR 7 BO 
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.t 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—I11. He 
tIt is qualified, however, by the admitted difference between the granitic and 
trappean rocks, the former being more prevalent at the earl t peri ds,—a differ- 
ence which is ascribed to a “ modification in the condition of things.” 
