MONIAN SYSTEM OF ROCKS. 



511 



one. But the purple rock is sedimentary and calcareous, it is 

 cracked in numerous lines, and the cracks are filled with calcite. 

 The green rock, on the contrary, is a spherulitic diabase, while the 

 purple rock in the immediate neighbourhood of the green is filled 

 with streaks of andalusite crystals. We thus 

 have before us the base of a lava-stream 

 which has broken the rock into fragments, 

 infused itself into their cracks, and moved 

 them along in its stream (see fig. 17) . Further 

 on, this green and purple mixture rises up 

 into a conspicuous mound, visible for several 

 miles, standing out against the horizon. Here 

 perhaps we have the surface of the stream, 

 for it has the aspect of a Cyclopean rubbish- 

 heap, with the fragments of both rocks buried 

 in comminuted dust and weathering out in 

 utter confusion. Nothing can compare with this 

 but the termination of a torn torrent of lava, 

 which has pressed and broken against its own 

 debris and mingled lava, agglomerate, and ash 

 in one frowning front. Still further south the 

 diabase assumes greater importance in bulk, 

 and weathers into some beautiful large sphe- 

 roids, the interstices between which are filled 

 with radiating zeolites, which weather into 

 the aspect of a coral. It finally becomes a 

 continuous ridge, which forms the promontory, 

 and passes over into the east side of the island 

 of Llanddwyn, but it by no means occupies a 

 constant position in the series. 



Starting again at the first exposure of vol- 

 canic rock, we find the following section 

 (fig. 18) — that is to say, the broken limestone- 

 slate is followed to the east by a boss of blue 

 slate, and that by a boss of limestone of the 

 Cerrig Ceinwen type. The slate is the most 

 Ordovician-lookiug slate I have seen anywhere 

 in the series, but the interest centres in the 

 limestone. Here it looks stratified, like that 

 at Cerrig Ceinwen, since it stands parallel to 

 the strike of the slate ; but further south its 

 mode of occurrence is not such as to suggest 

 stratification at all, but rather to supply a 

 problem difiicult of solution. In the next 

 outstanding boss (fig. 19), as seen from the 

 south, the main mass of the rock is purple 

 and green ash, not unlike in its structure to 

 other ashy rocks seen at Dinas Llwyd ; but in the centre there are 

 two lenticular patches of calcite, and two other masses whose base 

 is not seen, and round some of the edges of the calcite the ash is 



Q.J.G.S. No.175. 2 m 



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