176 



IGNEOUS ROCKS OF WALES 



Upper 

 Gallery. 



Wall-case 4, 



31 shows the scoriaceous character. 



33 shows the scoriaceous character somewhat conglome- 

 ratic. 



69 and 71 show brecciated and also porphyrinic struc- 



tures. 



The relation of the ashes to the other rocks is especially 

 well shown in section No. 2, between Castell and Little 

 Hill ; in No. 3, on the right of the Breidden Hills ; in No. 4, 

 on the top of Snowdon ; in various streaks in No. 5, on both 

 sides of Corndon Hill ; in No. 6, on the Carneddau ; in 

 No. 7, between Y Dduallt and Careg Aderyn, and on the 

 left of the felspathic trap that forms the summit of Aran 

 Mowddwy ; in No. s, on the left of Cacler Idris, beneath the 

 felspathic trap in the hollow of Llyn-y-Gader, and in No. 9 

 between the top of Arenig fawr and the road to Tyn-y- 

 Mynydd. It will be seen that they are marhe< 

 stratified. Rarely they contain fossils. Unde 

 Cader Idris and Aran Mowddwy they are chiefly felspathic, 

 sometimes brecciated and conglomeratic, and occasionally 

 calcareous. They are much intermingled with ordinary slaty 



rfectly 



sediments, as might be expected of volcanic ashes showered 

 into Silurian seas. On Aran and Cader Idris they are about 

 2,500 feet thick, the accumidated result of many eruptions. 

 Passing northward they thin entirely away. On the left 

 side of Arenig-fawr (section 9) the trap marked 98 is the 

 equivalent of that marked 94 and 97 on the Aran Mowddwy 

 and Y Dduallt section (7). There is no ash under 98 ; it 

 has thinned out ; but there are other ashes of later date 

 above 98 forming the summit of the mountain. 



The porphyritic traps, ashes, and conglomerates of Moel- 

 wyn (section a) are the general equivalents of those of Aran 

 Mowddwy and Y Dduallt. They consist of felspathic por- 

 phyritic lavas, and volcanic conglomerates and ashes, dip- 

 ping north at 



angles of about 45°. 



The fossils of the 



ordinary stratified rocks in which they immediately lie 



belon 



of Cynicht, &c, to the Caradoc or Bala series, on a high 



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