GEOLOGY 



and as the quartz which forms the grains is the hardest of the common 

 rock-forming minerals, the rock which has been evolved from the weld- 

 ing of those grains is of exceptional strength. The compacting of the 

 grains by pressure has been accompanied by the filling of their inter- 

 spaces with a deposit of silica, which has acted as a cement and has been 

 precipitated chemically from heated waters. 



The killas is traversed by veins of white quartz which not only 

 occupy planes of fissure but ramify in all directions amongst the rock 

 masses. The amount of quartz so distributed is enormous, and to its 

 destruction we owe the greater part of the shingle on our beaches. This 

 siliceous rock formed no part of the original marine deposit, but has origin- 

 ated at a subsequent period during the subterranean phase of the Palae- 

 ozoic formations in which it is enclosed. In those depths the formations 

 have been more or less saturated with thermal waters which circulate be- 

 neath the surface. As rocks are not absolutely impervious the entire mass 

 was constantly searched and subjected to a process of leaching. The re- 

 peated passage of heated waters over every particle of the entire rock mass 

 removed silica in solution and redeposited it by chemical precipitation 

 along lines of fissure, which are the main channels to which such waters 

 ultimately converge. Notwithstanding the marked contrast which exists 

 between the ' killas ' and the quartz veins, the silica of which the latter 

 are entirely composed enters so largely into the composition of the killas 

 either in the pure mineral of the quartz grains, or in chemical combin- 

 ation with other substances, as to form more than half of the total 

 material of the rock mass, so that the chemical relations of the veins and 

 enclosing rocks are of the closest nature. 



These veins moreover are not all of the same age. While some 

 have participated in the flexure and brecciation to which the ' killas ' has 

 been subjected, others are undisturbed and have evidently been formed at 

 a period when the movements had ceased. 



The downward digression of that pile of marine accumulations of 

 the ancient Palaeozoic seas not only involved them in the disturbances of 

 the terrestrial crust represented by their mechanical deformation already 

 described, but brought them in close proximity to those great subter- 

 ranean furnaces, the home of vulcanism, the presence of which is often 

 so painfully manifested by the effects of volcanic action. 



In that subterranean region the Palaeozoic deposits have been invaded 

 by enormous masses of molten rock, which have produced extensive 

 alteration on the killas within their vicinity, carrying their metamorphism 

 a stage further, whereby the slates have been converted into schists of 

 such an advanced stage of crystallization, that in some cases the sedi- 

 mentary deposits formed on the sea floor have been confounded with the 

 products of volcanic eruption. 



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