6 A SKETCH OF THE GEOLOGICAL STRUCTURE 



spaces in the now shattered arches with hot, viscous rock. 

 Finally, when the full forces that urged the granite upwards 

 were nearly spent, the whole mass round about the remains 

 of the Gondwana and calcareous rocks cooled and began to 

 solidify, and after spurts of vapour and molten rock, carrying 

 tin amongst other things, had marked the last expiring efforts 

 of the magma the granite became a rigid mass. 



But now, after the Gondwana and calcareous rocks had 

 been buried for so long deep in the earth's crust, the rocks that 

 formed the surface were being fast worn away by denudation,; 

 until in the course of time the remains of the arches, and the 

 solid granite were close to the surface. Before denudation 

 brings them to the light, let us examine their arrangement in 

 more detail. 



Dislocation on a large scale had destroyed the former 

 continuity of the rocks composing the arches, and large blocks 

 had dropped down into the magma, their place being taken by 

 granite. The arches had in all probability been forced up- 

 wards to some extent when the pressure from the magma was 

 first felt, but when their stability had been completely destroy- 

 ed the dislocations were caused by downward movements. 

 They were what are known as "faults," and they so affected 

 the beds with which we are immediately concerned that 

 Gondwana rocks dropped down so as to be side by . side with 

 the calcareous rocks. The main dislocations occurred along 

 lines trending roughly N.N.W.-S.S.E., but there is evidence of 

 other dislocations at right angles to these lines. In the por- 

 tions of the arches near those that the granite had broken 

 through, that is near the weakest places, the close folding of 

 the rocks under pressure had caused the bedding of both series 

 to appear vertical although had it been possible to bore through 

 the vertical Gondwana beds in any one block we should 

 have come to vertical beds of calcareous rocks below. In both 

 series there were rocks of varying hardness. The conglom- 

 erates and quartzites of the Gondwana rocks were harder 

 than the clay slates of the same series, the calcareous clay 

 slates were not so hard as the limestone that had been 



Jour. Straits Branch 



