CARBONIFEROUS LIMESTONE OF CARMARTHENSHIRE. 



157 



to vitrification. On tracing, however, these glazed fragments up the bed of the moun- 

 tain rivulet near Blaen Cennen, in which they had been found, I discovered their origin 

 to be in a portion of the great escarpment, which having been subjected to a downcast, 

 is for the space of a mile almost submerged in a turf bog, the ends alone of the strata 

 being exposed in the bank of the little stream. (See Map and PI. 30. fig. 12.) From the 

 unaltered condition of the limestone and shale underlying these glazed beds, it appeared 

 almost certain, that the altered external characters of the upper layers of the rock, could 

 not be due to any cause connected with igneous or gaseous agency acting from beneath • 

 and besides no rock of trappean or volcanic origin occurs in any part of the adjacent 

 tract. Internally, these beds present the usual semi-crystalline structure and grey colour 

 of the limestone, but the varnished coat extends over all the surfaces, including every 

 cavity. The glaze is brittle, yields to the knife, and is colourless and transparent, the 

 veins of calcareous spar and tints of the rock not being hidden by it. As these glazed 

 strata, protruding from beneath a deep peat bog, have an aspect similar to that produced 

 on limestone by very diluted muriatic acid, it has occurred to me that the peculiar con- 

 dition of the surface might have resulted from long-continued immersion in water, im- 

 pregnated with the slight vegetable acid of the bog. 



Caverns frequently occur in this limestone, and that from which the river Lwchwr 

 suddenly bursts forth is a fine example, analogous to those sources of subterranean 

 streams which are of such frequent occurrence in every country where calcareous rocks 

 abound. 



The most prevailing fossils are the 



Productus hemispharicus. M. C. t. 328. 



Martini. M. C. t. 318. figs. 2, 3 and 4. 



comdtdes. M. C. t. 329. 



together with many prevailing corals of the formation, all of which are dissimilar from 

 any which we shall afterwards describe in the Silurian System. (See detailed list of 

 many of these fossils at the end of this chapter.) 



In its course through Caermarthenshire, westward of the great dislocations near the 

 Fans, the zone of limestone maintains a considerable thickness as far as the pass of 

 Llandybie, and at the extensive limeworks of Garn-fach or Calch-gorog, four miles south- 

 east of Llandeilo, I estimated it at not less than three hundred feet. Thence to the 

 fiexure which forms the extreme south-western edge of the coal basin, the zone becomes 

 thinner, and the large quarries of Llangyndeyrn, six miles south-east of Caermarthen, 

 are the last spots where it is seen in force; for to the south-west of the Mynidd Garreg, 

 and immediately north of Kidwelly, the rock dwindles to an obscure band, which is no 

 further traceable in the sea cliffs on the left bank of the Towy. By reference to the map 

 it will be perceived, that further to the westward, patches of this carboniferous lime- 

 stone re-occur on the coast near Laugharne, overlying the Old Red Sandstone in both 

 banks of the River Taaf. Although these patches are in Caermarthenshire, they form 



u 



