COAST SECTION OF LOWER SILURIAN ROCKS. 



393 



an irregular boss of trap rock which here rises into the cliff. This trap will be described in the sequel. In a few paces 

 beyond this intrusion the regular succession is renewed. 



8. Dark grey shale with calcareous concretions, some of which pass into thin courses of grey limestone. On the surface 

 of these are many fossils belonging to the Wenlock limestone, including Terebratula affinis, Orlhis radians, Atrypa aspera, 

 Spirifer lineatus, S. radiatus, and a new species found at Dudley. Portions of Trilobites, Crinoidal stems, Orthoceratites 

 with remote joints, unnamed conical tubes occurring at Dudley, together with many of the Corals of this formation, and some 

 very peculiar organic bodies which will be described in subsequent chapters. 



These fossils clearly prove, that the band No. 8 is the equivalent of the Wenlock limestone, and hence the overlying strata 

 from 1 to 7 inclusive must represent the Ludlow rocks, and although they are not charged with as many organic remains 

 as in Salop and other parts of their range, yet they here occupy a considerable zone. 



The Wenlock shale with nodules and fossils, as exhibited in a little cove, is underlaid by other sandstones and shale 

 No. 9, which from their imbedded fossils belong to the Lower Silurian Rocks. 



Such are the rocks representing the upper portion of the Silurian System in these 

 coast cliffs. They range inland across the promontory of Marloes. 



Lower Silurian Rocks. 



We cannot better commence the account of these rocks in Pembrokeshire than by 

 continuing the instructive section, in descending order, afforded by the cliffs of Marloes 

 Bay. 



9. The strata which rise from beneath the grey shale with Wenlock corals, and occupying the coast at Marloes Mill, 

 consist of hard, thick-bedded, grey sandstone, with white quartz veins and shale, followed by other sandstones somewhat 

 calcareous, having on their surfaces ripple marks, and their mass is traversed by veins of white calcareous spar. 



10. They pass downwards into sandy schists, which are abundantly charged with casts of the fossils characteristic of the 

 Caradoc sandstone, such as the well-known coral of May Hill and other places, with numberless circular impressions of the 

 ends of encrinital stems. 



These beds, as well as those of the overlying Wenlock shale, are impressed with a 

 transverse slaty cleavage, which cuts through and so distorts the fossils, that fragments 

 of these rocks might be referred to strata of as high antiquity as the rocks of Snowdon. 

 Attention has been previously directed to similar changes in the lithological aspect of 

 rocks of this age near Caermarthen, and such examples, we repeat, ought to act as 

 powerful warnings against all attempts to identify the age of formations by their external 

 characters. To the west of Marloes Mill is a fault by which the Old Red Sandstone of 

 the Isle of Gateholm, with a small tongue of the same on the mainland, is brought into 

 contact with these Caradoc sandstones, thus : 



Isle of Gateholm. Stack. Horse's Neck. 79. 



Marloes Bay. 



a. Old Red Sandstone. b. Caradoc Sandstone with slaty Cleavage. 



