29 



way with the kva poured out in Iceland in 1793 it would be very unpbUosophical 

 to take for granted that the effects of subterranean heat surpassed at remote 

 eras the corresponding effects in our own times. The trap rock in this district 

 differs very much in its character at different places, as it usually does. 



The stratified rocks in connection with the trap are for the most part the 

 lower Silurian rocks-the LlandeUo flags, tilestones, and shales, and xn the 

 Llandegley Hills the Caradoc sandstone ; but in the dingle below (the black- 

 smith's dingle), leading to the Rock House Hotel, the Wenlock shale crops up, 

 and in the valley of the Ithon the perishable mudstone of the upper Sdunan 

 group appears. The subsoil of the valleys mostly consists of black shale, schist 

 and sandstone, mixed with the clay detritus from the trap above, but the subsoxl 

 of the higher lands-like this Common-is composed chiefly of a cold yeUow 

 clay from the decomposition of the felspar of the trap, and though interesting 

 to botanists, perhaps from the plants which prefer it to a better soil, it presents 

 to the agriculturalist a barren and unfertUe district. 



The quarries at Llanfawr which you have just visited, show well a highly 

 crystalline greenstone, in conjunction with the Llandeilo shales, whose strata 

 have been thrown into different angles by the trap eruption, some dipping due 

 North and others West. The greenstone is there composed of hornblendes 

 and grey felspar more or less crystalized. The same rock appears near the 

 weUs. The hiUocks around the church are amygdaloid, with kernels of quartz, 

 sometimes coated with anthracite. These and other varieties of trap protrude 

 throu<rhout the district, in almost countless bosses, and wherever they occur 

 the black shale, the schist, or flagstones between them are highly indurated, 

 dislocated, altered in structure, and always more or less contorted. 



At one of these points of contact the Mineral Waters of Llandrindod 



issue from the black shale. Sometimes, as at Llandegley and Builth the 



stratified rocks have been so contorted as almost to stand on end, and the 



interstices between them, through which the springs issue, may be compared 



to the conduit pipes of an artesian well, with this difference, that the waters 



themselves receive additional impregnation from the rocks they pass through. 



The structure and mineral constituents have been much altered by the intense 



heat to which they were exposed, but less so, as a matter of course, as they 



lie more distant from the molten trap. In this way is explained the varying 



character of the springs, as they issue, perhaps very near to each other, from 



different strata of the shale or Flagstones. Those containing most sulphur 



are usually nearest the trap rock. Then come the strong salines and chalybeate 



springs, which become weaker in their qualities as they approach the unchanged 



rocks. This description applies more forcibly to BuUth than Llandrindod, where 



the sources are somewhat further apart. The rocks generaUy contain sulphuret 



of iron, but Sir Roderick Murchison has never seen it in such "crystallized 



bunches" and large " flat nodules, an inch or an inch and a haH in diameter," 



as in the altered rocks at their junction with the trap. 



