358 



BASE OF THE SILURIAN SYSTEM.— DISLOCATIONS. 



the left bank of the Towy. From this point to the south and west, the formation contains no cal- 

 careous masses till we reach Clog-y-frain, on the borders of Pembrokeshire, a distance of about 

 fifteen miles. Its course however is marked on the left bank of the Towy at Pont pibwr, opposite 

 Caermarthen, by trilobites, which occur in nodules in black argillaceous rotten shale, (see Section, 

 PI. 34. f. 9.) passing down into strata of black schist, containing at Mount Pleasant, casts of several 

 shells, most of which are greatly contorted. 



I consider these fossils, like those west of Noeth-griig, to occupy the base of the 

 Llandeilo formation. Among them are several of new species not mentioned in previous 

 chapters, such as Nucula Icevis, Spirifer alatus, Euomphalus perturbatus, and Ogygia 

 Murchisonce 1 , PL 25. figs. 3 a & 3 b , together with the well-known fossil Orthis Flabellula, 

 and others common in the Lower Silurian rocks. 



Between the Towy and Taaf, and thence extending by St. Clears, the black shivery 

 schists are supposed to represent the Llandeilo flags, although they contain no calca- 

 reous beds ; or, as far as I could observe, any very characteristic fossils. But at Clog- 

 y-frain to the west of St. Clears, and on the left bank of the Towy, a prominent ridge 

 of limestone rises from the midst of black schists and flags. In this ridge are three 

 calcareous bands, separated by calcareous flags and shale, making in all a thickness of 

 upwards of 200 feet : the uppermost band of limestone being of very fine quality, and at 

 least seventy feet thick, has been the most largely quarried. The beds are, therefore, much 

 thicker than those of any mass of limestone of this age previously noticed. One of 

 these, about ten feet, is in fact a complete aggregate of shells, corals and trilobites, and 

 is of a dark-bluish grey colour, crossed by white veins, which mark the course of the 

 joints. Some of the beds are so charged with crinoidea, that they might be well termed 

 encrinite limestones ; and among these remains is one having pentagonal joints. Orthis 

 flabellula, 0. alternata, 0. canalis, and 0. bilobata are the prevailing conchifers. Corals 

 are very abundant on the surface of the beds, and Asaphus Buchii is not unfrequent. 

 This beautiful marble rock passes up into sandy limestone, and the calcareous matter 

 diminishing, is soon lost amidst the surrounding shale and sandstone, in the form of a 

 concretion. 



The variations in the strike and dip of the beds are proofs of the powerful disturb- 

 ances by which they have been affected. The strata bend round from W.N.W. to 

 E.S.E., to N.W. and S.E., the last strike being absolutely at right angles to the general 

 direction of the formation in South Wales ; and the dip, instead of being to the south- 

 west, so as to pass under the superior formations, is completely reversed, or thrown 

 over to the north-east at angles of 55° and 60°. On examining the adjoining gorge of 



1 The principal trilobite figured from this locality is a new species of Ogygia, which I have named after 

 Mrs. Murchison, who unexpectedly discovered this fossil in a locality where I had abandoned the hope of 

 finding one. The trilobites and fossils of this spot, together with those of Llandeilo, bear so strong an analogy 

 to those described by M. Brongniart from Angers in France, that I have little doubt of the black flags of the 

 latter place proving to be of the same age as the bottom beds of the Silurian System. 



