VOL. XVII. (i) EXCURSION— BUILTH-WELLS & RHAYADER 21 



The principal rocks taking part in the formation of the Carneddau are : " 



(i) Diabases, which are intrusive in the Upper Llandeilo Shales, but 

 pre-Silurian ; 



(2) Diabase porphyrites which are readily recognized by their coarsely- 

 crystalline structure ; 



(3) Rhyolites, which weather very white ; 



(4) Andesitic ashes, which are extensively worked for road-metal in 

 the quarries north of Llanelwedd, and have yielded sparingly 

 Orthis calligramma, Dalm., and a few other fossils; and 



(5) Andesites. The quarry by the road-side, just before reaching 

 Gelli Cadwgan, is in this rock, and was opened for obtaining 

 road-metal. 



The diabase (i) is well exposed in a quarry by the side of Pencerig Lake, 

 and again in a road-cutting at the back of Pencerig House ; while masses of 

 this rock cross the Wye between Builth- Wells and Builth-Road Station, and 

 are the cause of the "' Wye Rapids " (PI. IV., fig. 2). 



The Upper Llandeilo Shales are well exposed in a quarry by the road- 

 side near " The Rapids," where numerous specimens of Ogygia Biichii 

 (Brongn.) may be collected ; in Harper's Quarry, where the numerous speci- 

 mens of Ogygia Buchii (Brongn.), Tnnucleus fimbriatits, Murchison, Ampyx 

 nudus (Murch.), and Siphonotreta micula, M'Coy, are beautifully preserved in 

 hard greenish rock ; and again at Trecoed. 



No Bala or Lower Llandovery rocks appear to be present in the Builth 

 district, and the Upper Llandovery Pentameyus-Grit follows the Llandeilo 

 Beds at once. 



This Upper Llandovery Grit, of a yellowish colour and not unlike 

 the fine-grained parts of the Upper Llandovery or May Hill Sandstone 

 of May Hill, is to be seen in the bed and south bank of the Wye between 

 " The Rapids " and the rail way -bridge, and again at Trecoed. 2 It is more 

 accessible at this latter place, where from four to six feet of it is seen 

 separating the black Llandeilo Shales from the succeeding Wenlock Shales of 

 the Cyrtograptus-Mttrchisoni-Zone. Entering the gate of the field opposite 

 the entrance to Trecoed, the Upper Llandovery Grit will be at once 

 perceived, with some purplish shales (? = the Tarannon Shales) on top ; 

 while below are the Llandeilo Shales. They are best exposed in the bed 

 of the stream, but in 1909 there was a considerable heap of shale, rich 

 in specimens of Ogygia Buchii, that had been thrown out of an excavation 

 for a well. At the Llanfawr Quarry, Llandrindod Wells, the Llandeilo Shales 

 are magnificently displayed, and nearly every slab that is detached is covered 

 with trilobites (mostly Ogygia Buchii) and graptolites. 



From their outcrops, marked by the Wye and Trecoed sections, the 

 Upper Llandovery Grit and probable equivalent to the Tarannon Shales, dips 

 down in a westerly direction beneath the Ludlow rocks. When they reap- 

 pear, the Tarannon Shales are found to have passed into the huge mass 

 of pale shales called the Rhayader Pale Shales, and the Grit into the massive 

 Caban Conglomerates ; while below have come in the Gwastaden Group and 

 beds probably referable to the Bala Series. But of this matter more anon 

 (see page 23). 



From " The Rapids " the Members drove on towards Trecoed, seeing on 

 the way the tumulus and ancient Cwrt-Llechrhyd intrenchments, and the 

 richly-fossiliferous Linnarssoni-Zone Shales by the road-side close to Builth- 

 Road Station, but then, owing to heavy rain, the visits to the Dulas Brook, 

 Coed Mawr, Castle Crab, and Trecoed sections had to be abandoned. 



1 See H. Woods, Quart. Joum. Geol. See, vol. 1. (1894), pp. 566-577. 



2 See Proc. Geol. Assoc, vol. xix. (1905), p. 229. 



