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ON THE GEOLOGY OF THE LLANDRINDOD DISTRICT ; 



ITS MINERAL SPRINGS ; AND CONGLOMERATE 



BOULDERS. 



By T. CURLEY, Esq., C.E., F.G.S. 



Gentlemen, — The simple outline of tlie bold undulating hills, seen from 

 all parts of this Common, would be enough to prove that this district has been 

 the theatre of great volcanic action ; but the further you inquire into the 

 subject, the more you examine the rocks themselves, the greater will be your 

 wonder at its magnitude and its incalculable power. The shock which broke 

 through the vast thickness of stratified rocks previously deposited here not 

 only allowed the large mass of molten igneous rock, which now forms the 

 Llandriudod Hills we stand upon, the Coed Mawr, the Carneddau, and other 

 hills of the district to pass through the fissures, but it so shook and separated 

 the strata that the liquid rock poured in between them, and became bedded 

 itself. At the same time, too, the astounding force was produced which raised 

 those enormous masses of bedded trap and stratified fossiliferous rocks together, 

 which form the large and lofty hills of Llandegley, of Gelli, and of Gilwern. 

 We are now surveying a considerable portion of the largest trap district in 

 Radnorshire, a district well described by that eminent man, who first read the 

 true character of the Silurian rocks— Sir Roderick Murchison. It extends 

 from Llandegley and Llanbadarn-fawr on the north and north east, to the 

 environs of Builth on the south west, and is about ten miles long and five 

 broad. From Llandegley to Builth the main ridges of trap are the Llandegley 

 Rocks, Sunny Bank, Gelli, and the Carneddau. Besides these a great entrench- 

 ment is thrown out, which concerns us to-day. It is on the north-west, by 

 Carreg-gwiber, and Cwm-brith-hill to Cefn Llys, and the hillocks north of Llan- 

 driudod Wells. There are also others out of our present field. 



Many of these trap ridges inclose longitudinal valleys, running from north- 

 east to south-west. They are excavated in the lower Silurian shale and flags, 

 the beds being tilted in divergent directions upon the opposite sides of the 

 intrusive rock. Many of the shales and flags are very much altered in character 

 at the points of contact with the trap. They have undergone considerable 

 changes in their mineral character, from the intense heat evolved, and thus 

 veins of quartz, and occasionally metals (lead in this district), coats of anthra- 

 cite, cavities filled with green earth, hardened silicious rock, &c., &c., have been 

 produced. 



The period at which these great changes occurred it is impossible to 

 define. Igneous rocks have been produced by volcanic action at all geological 

 periods. Similar trap rocks to these can be shown to have been thrown out 

 both before and during the formation of the Laurentian, Cambrian, and the 

 Carboniferous rocks. If we compare the columnar basalt of the Giant's Cause- 



