1853.] RAMSAY — LOWER PALiEOZOICS OF N. WALES. 163 



part of Cader Idris is composed of two masses of felspathic trap, be- 

 tween which there lie various interstratifications of slate, greenstone, 

 and felspathic ashes. Where the section crosses the mountain, the 

 lower felspathic trap lies on the north side of the cliff, and the other 

 to the south-east of Llyn-Cae on the slope towards Tal-y-Uyn. The 

 slaty beds are hardened at the points of junction, and, though the 

 traps appear to be perfectly interbedded, yet they are certainly intru- 

 sive, seeing that, though here separated from each other by about 

 2000 feet of rocks, they coalesce at short distances both on the N.E. 

 and S.W. The same holds true of the greenstones. 



The Bala and Llandeilo beds overlie these rocks, passing across 

 the country south-east, in a series of rapid contortions, to the neigh- 

 bourhood of Llanbrvnmair, where the Caradoc sandstone comes in in 

 a small trough, which is the equivalent of the first larger undulating 

 trough of Caradoc sandstone in the country east of Aran Mowddwy, 

 as shown in Section No. 2. The Bala beds are repeated on the east of 

 the Llanbrynmair trough, and are again capped by Caradoc sandstone 

 on the west slopes of Pegwns-fawr ; beyond which, as far as the 

 neighbourhood of Radnor Forest, the Caradoc sandstone, with troughs 

 of Wenlock shale, is spread across the country in numerous anticlinal 

 and synclinal axes. These troughs are all laid down on the Survey 

 maps. In the area between Cader Idris and Llanbrynmair no 

 fossils have been discovered. The author found a few Graptolites 

 in equivalent slates near Machynlleth, and further south, at the 

 Devil's Bridge, the Bala fossils found by Professor Sedgwick are well 

 known. 



The Cader Idris traps are continuous with those of Aran Mowd- 

 dwy ; and in that direction, after the two felspathic bands of Cader 

 Idris join, it continues on one geological horizon to the northwards 

 perfectly interstratified with the aqueous rocks of the country. The 

 slates that underlie it at the Arans are porcelanized, and those that 

 overlie it are unaltered ; whence it is believed that the intruded rocks 

 of Cader Idris pass northwards into what was once a true contempo- 

 raneous felspathic lava-flow. The Bala beds succeed the trap of the 

 Arans. The Bala limestene is about 6000 feet above the trap, and 

 between the limestone and the Caradoc sandstone on the east there 

 are about 8000 feet of slaty beds, mingled with occasional sandstones. 

 East of the Arans, therefore, the whole Bala or Llandeilo series 

 seems to be about 14,000 feet thick. The Caradoc sandstone that 

 overlies it is not less than 5000 feet thick. It seems to rest conform- 

 ably on the Bala beds. This gives in thickness for all the rocks above 

 described : — 



Cambrian 6,000 



Lingula and Bala beds, with igneous rocks 

 interstratified 26,000 



Caradoc Sandstone 5,000 



42,000 feet of 

 rocks seemingly deposited conformably on each other, the whole 

 having been at later periods contorted and faulted together. 



The ashes below the Aran traps are the equivalents of those of 



