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old lake. The rocks of the Carneddau Ilills, that form so picturesque a 

 feature near Bailth, are cornposeJ of igneous rocks both eruptive and stratified, 

 and these run to the nortli east to Lhvudrindod and Lland 'gley. In many 

 sections in the neighbourhood the old lava currents may be seen elevated 

 through the overlying sedimentary deposits, and throwing off the Llandeilo 

 strata, and intorbedded with flagstones which contain the well known Llan- 

 deilo trilobites. 



Perhaps the most instructive section is that typical section given by Sir 

 R. Murchison, in "Siluvia," viz., that on the N. W. slope of the Gelli Hill, 

 where felspar porphyries, like those of North Wales, may be seen inter- 

 Btratiasd with Llandeilo flags, containing that typical trilobite the Ogygia 

 Buchii. 



Another section, very instructive, is that on the Wye, above the cele- 

 brated salmon catch, called the Rocks. Here, the Llandeilo strata may be 

 seen broken through and altered by the volcanic masses and worn down in the 

 river bed. The fisherman can always tell when he is wading over the igneous, 

 or the stratified rocks, the latter being so much more slippery than the rough 

 lava beds. 



Another feature the physical geologist will not fail to remark, is the 

 excavation of the softer Llandeilo shales, and the longitudinal valleys scored 

 out by denudation, the hard trap rocks standing out in hill ridges. This tells 

 its own history. The mineral waters of Builth, Llaudrindod, and Llandegley, 

 owe their origin to the decomposition of iron pyrites at places where a lava 

 rock traverses and alters the Llandeilo strata. The waters of Moffat, in Scot- 

 land, known for their chalybeate propei ties, take their rise through a fissure 

 of Llandeilo strata charged with graptolites and altered by an ancient lava 

 dyke, jirecisely as occurs in this district. I have been asked if it was pos- 

 sible to point out the site of the ancient volcano from which the igneous 

 rocks of the Builth districts were poured forth. It must be remembered that 

 much of the old lava was evidently evolved through fissures into the bed of the 

 Llandeilo sea, for they were soon covered up by sedimentary silts and strata. 

 Still, the volcanic ash of some of the deposits tell of a volcano which must 

 have had its crater above the waves to have showered forth such light materials 

 into the air. We must, however, remember the total submergence, of this 

 country more than once since the period of Llandeilo volcanos, and that in all 

 probability we only now behold the basement rocks of those volcanic masses 

 which once existed along this line of ancient earthquake action and volcanic 

 eruption. 



Long after the lava rocks of Carneddau had become as hardened and as 

 cold as they are now, volcanic action burst forth again, and molten masses 

 were poured through rocks of a much later date only a few miles from 

 hence. I allude to the well-known trap rocks of Stanner and Old Radnor, 

 which on the north of Builth traverse and alter rocks belonging to the 

 Upper Silurian epoch, the Woolhope or Lower Wenlock beds. I have no 



