196 



HA RD WICKE' S SCIENCE- G OSSIP. 



of changes. They may, perhaps, be depressed, until, 

 coming within the influence of the earth's internal 

 heat, they are fused and converted into a pasty or 

 liquid mass. In this condition they may be acted 

 upon by causes which result in their being ejected 

 from volcanic vents, or forced into rents and fissures, 

 forming lava streams and dykes, and then they would 

 be regarded as truly igneous. But are they not also 

 metamorphic ? If, as I have supposed, these rocks 

 resulted from changes wrought upon sedimentary 

 strata — and there is no reason for denying that such 

 changes could take place — then I think they may 

 truly be claimed as metamorphic, having undergone 

 change. But if the term metamorphic be restricted 

 in its application, and defined, let us say, as meaning 

 those rocks only which result from the change of 

 other rocks, but are never eruptive, then, although it 

 cannot be conceded that all granites are metamorphic, 

 since some of them can be proved to be eruptive, yet, 

 at the same time, I do not think it can be denied 

 that some granites, at least, owe their origin to 

 metamorphism. Now to glance at the evidence 

 afforded by granite itself. 



As we have already seen, granite is a perfectly 

 crystalline rock without any glassy mass or crystallites. 

 This is a distinctive character, and proves, by reference 

 to the glassy rocks, that it has cooled down/rom fusion 

 very slowly. Lavas which have cooled rapidly on 

 the surface of the earth, consolidate as true glasses. 

 But when the cooling process has been retarded, 

 time has been given for chemical affinity to act, and 

 cause the separation of certain portions having a 

 definite composition ; and in this way microliths and 

 crystallites, or incipient crystals, have been formed. 

 In proportion as this retardation in solidification has 

 been great, so have these crystallites been more and 

 more perfectly developed, the rock at the same time 

 losing its glassy character as it becomes increasingly 

 filled with crystals and crystalline matter. 



When rocks are artificially fused and allowed to 

 cool rapidly, they become glassy ; if the cooling 

 process be prolonged, they take up a stony character 

 and may develop crystals. But it is impossible that 

 granite can have consolidated from a state of fusion 

 that can be imitated by artificial means. In the first 

 place it is found that the constituents have not 

 solidified in the order of their fusibility. Quartz, 

 the most infusible of the constituents of granite, 

 would naturally be expected to be the first to 

 crystallise on the cooling down of the granitic magma. 

 But it has in fact been the last, and is found to 

 envelop the crystals of mica and felspar, both of them, 

 under normal conditions, much more fusible than 

 quartz in a crystalline mass without having itself 

 taken on any definite crystalline form. Again, 

 granite occasionally contains such minerals as gado- 

 linite, orthite, and allanite, which, when heated to 

 dull redness, lose their physical characters. From 

 these facts it is inferred that granite could not have 



solidified from simple igneous fusion, or from such as 

 can be artificially imitated. 



By a microscopic examination of the quartz of 

 granite, a clue is given to the true explanation. The 

 quartz is found to contain an enormous number of 

 cavities, more or less filled usually with water con- 

 taining' chlorides or sulphates of sodium and potassium. 

 Sometimes they contain liquefied hydrocarbons or 

 carbonic acid. Speaking of these cavities, J. Clifton 

 Ward says, "A thousand millions might easily be 

 contained within a cubic inch of quartz, and some- 

 times the contained water must make up at least five 

 per cent, of the whole volume of the containing 

 quartz." The presence of these liquids proves that 

 granite has consolidated under enormous pressure 

 and in the presence of water. The fusion, then, was 

 not simple dry fusion, but was accompanied by 

 abundance of superheated water. It is most probably 

 to the effects of this superheated water that the 

 anomalous conditions of solidification may be due. 

 The pressure under which the rock solidified would 

 be greatly increased by the expansive force of the 

 contained water, and this added to the weight of the 

 superincumbent earth, and the effect of secular con- 

 tractions of the globe, would be indeed great. It has, 

 in fact, been calculated by Sorby that some granites 

 have consolidated under a pressure equal to the 

 weight of 50,000 feet of rock. 



Granite is found occurring as a true volcanic rock 

 and as eruptive masses. In cases where denudation 

 has laid bare the ancient reservoirs of volcanoes, granite 

 is exposed to view, taking the place of that once-fluid 

 mass from which the acid lavas were supplied. Huge 

 bosses of granite rising through, and more or less 

 altering the sedimentary strata, again attest the 

 igneous origin of granite. In some places the 

 granite has not only pushed aside the stratified 

 rocks in its ascent, but has actually caused their 

 disappearance, as if it had melted and absorbed 

 them into itself. In the south-east of Ireland, for 

 instance, there occurs a belt of granite which has 

 eaten its way up through the Cambrian and Silurian 

 rocks, absorbing much of them into its own mass as 

 it rose. Within the margin of the granitic belt 

 numerous patches of schist are taken up, and the 

 surrounding rocks are altered into mica-schist for a 

 considerable distance, and are pierced by veins given 

 off from the main mass. 



With respect to granitic veins, it may be here 

 remarked that they vary considerably in texture, 

 some being coarsely crystalline whilst others are 

 fine-grained. They vary also in breadth, from mere 

 filaments to several yards in thickness. Their effect 

 upon mineral veins which traverse them is referred to 

 by De la Beche, in his "Geological Observer," 

 where he observes, speaking of the load at Wheal 

 Alfred, Gwinear, in Cornwall : — "While the fissure 

 traversed the upper and adjoining slate, on the north 

 no great amount of ore was obtained, but upon 



