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. 23
