398 



CAMBRIAN ROCKS OF PEMBROKESHIRE. 



about a mile north of Haverfordwest, where on the right bank of the river it was formerly quarried 

 for burning. It is there a bulging concretionary mass, much less pure than at Llampeter and Llan- 

 dewi, and rises in an arch from beneath the Caradoc sandstone of Prendergast and Haverford. 

 Fossils are apparently much scarcer, though I perceived fine specimens of corals. To the west of 

 Sholeshook we lose all distinct traces of the Llandeilo flags, as marked either by fossils or limestone; 

 but the black schists occasionally passing into flagstones, which are continuous from that place by 

 Crow Hill, Red Hill, and Cutty Bridge, most probably indicate the course of the formation, since at 

 Camrose we again meet with a black limestone, which, though apparently void of organic remains, 

 agrees exactly in mineral characters with the Llandeilo limestone of other parts. Here the whole of 

 the Silurian System gradually bends round from a westerly, to almost a northerly direction, in which 

 strike the calcareous flags and limestones of Camrose dip 40° west, and may, therefore, be supposed 

 to pass beneath the sandy incoherent schist and sandstone, which represent the overlying members 

 of the Lower Silurian Rocks. This gradual attenuation of calcareous matter to the westward in 

 the inland range of the Llandeilo flags, precisely accords with the features in the sea cliffs of St. 

 Bride's Bay, where the formation is seen for the last time containing the true organic remains, but 

 very little lime, (see p. 394.) 



Cambrian System. 



A tolerably precise base line of the Lower Silurian Rocks may be drawn along the 

 northern flank of the above-mentioned Llandeilo formation, as defined by the cal- 

 careous flags and limestone with associated fossils. The next inferior strata, like a 

 large portion of those in Caermarthenshire, are, however, with difficulty separable by 

 their mineral aspect from the overlying beds of the Silurian System, except where cer- 

 tain bands of encrinital sandstone occur similar to those described, (pp. 394 and 395.) 



a. The upper beds consist of a vast development of black shivery schist, generally void of hard stone 

 bands, with no traces of lime, and with scarcely a vestige of organic remains. In some districts they 

 form more naturally the base of the Silurian System, into which they graduate, but in Caermarthen- 

 shire they also pass into true roofing slates \ it is therefore preferable to consider them the beds of 

 passage between the two systems. To the north of the escarpment they dip under and pass up into 

 well recognised Llandeilo flags through the medium of certain beds of grey and brown grit, similar 

 to those on the north-western face of Grongar Hill, Caermarthenshire. Owing to their slight and 

 variable inclination, these schistose beds in Pembrokeshire occupy a wide zone between the true 

 Silurian System on the south and the hard Cambrian rocks on the north, but owing to their 

 perishable nature they have been much denuded. Rising to little altitude, and weathering to a stiff 

 clay, they present a cold, barren, agricultural surface, strongly contrasted with the adjacent fertile 

 tracts occupied by the Llandeilo flags. These schists are nowhere exposed in the coast of St. Bride's 

 Bay, being cut off by the trap which ranges from Castle Rock to Trafgarn, and overlapped still further 

 to the west by the coal measures of Newgale Sands and Brawdy. 



b. The shivery schists are succeeded on the north by courses of hard sandstone, grit, and thick 

 flagstone, all of which agree, more or less, mineralogically with the term "greywacke." Examples 

 of this rock occur at Parkstone quarry, eight miles north of Haverfordwest, where there are hard 

 intractable grits and sandstones, of light grey colour, made up of fragments of quartz, trap, slate, 



