228 



ferous limestone, and the lowest members graduate into the Silurian 

 system. The great mass consists of sandy shale, here termed the "red 

 rab," associated with red sandstones and grits ; but lithological varia- 

 tions from the usual types in Herefordshire have led to the belief that 

 large districts (Cosheston, Williamston, Benton, &c.) consisted of 

 greywacke. These are yellow, grey, and greenish micaceous sand- 

 stones which the author proves to be interlaced with the " red rab," 

 and to occupy the same position as similar varieties of the rock pre- 

 viously described in Hereford, Radnor, &c. Some of the coarse grits 

 (Canaston wood) are undistinguishable from the " greywacke " grit 

 of the oldest rocks of the Cambrian system. Calcareous matter is 

 very sparingly exhibited, imperfect concretions or very impure " corn- 

 stones" appearing only at wide intervals. The fishes so profusely de- 

 tected by the author in the range of the formation through Salop, 

 Hereford, and Monmouth, have not been observed. Amid the many 

 faults affecting this formation, those by which the strata ranging from 

 Caermarthen into Pembrokeshire have been powerfully bent and bro- 

 ken, and thrown into a westerly direction (Tavern Spite, &c.), are 

 perhaps the most striking. 



Silurian System. — Though the order of superposition and the or- 

 ganic remains clearly attest the age of the rocks of the Silurian system, 

 the masses differ so much in mineral aspect from those selected as 

 types that it is rarely possible to subdivide them into the Ludlovv, 

 Wenlock,Caradoc, and Llandeilo formations; but adopting the classi- 

 fication proposed, the author has laid down their course upon the map 

 as two sub-groups consisting of ^' upper and lower Silurian rocks*." 



The former parting with their mudstone characters are for the most 

 part hard and siliceous, containing little calcareous matter, and are 

 never subdivided by zones of limestone as at Aymestry and Wenlock. 

 The lower Silurian rocks, on the contrary, are amply displayed in all 

 their characteristic forms, the limestones of the Llandeilo formation 

 expanding to greater thicknesses (Llanpeter-Felfry, Llandewi, &c.) 

 than in any other part of their course, and containing many beautiful 

 fossils, including two unpublished species of Trilobites, common to 

 Caermarthenshire. The chief mass of the Silurian system ranges from 

 E. to W. across the county, passing by Haverfordwest, till its western 

 extremity subsides beneath the coal measures of Druson Haven and 

 St. Bride's Bay. Other bands of it rise from beneath the old red 

 sandstone at Orlanton, Hoten, and Johnston ; whilst a most re- 

 markable zone is heaved up in an anticlinal line extending across the 

 most southern promontory of the county (Castle Martin Hundred), 

 from Fresh Water East to Fresh Water West. The most perfect 

 succession of the rocks of which the system is composed, is exhi- 

 bited in the bold coast cliffs of Marloes Bay, extending for a distance 

 of two miles, in which space the uppermost strata, rising at angles 

 of 35° to 40° from beneath the old red sandstone of Hook Point, 

 are succeeded by conformable, underlying masses, until the whole 

 graduates down and passes into the rocks of the Cambrian system 

 in Wooltack Park and Skomer Island. The Ludlow and Wenlock 

 * See Lond. & Edinb. Phil. Mag., Vol. VII. p. 46, Silurian System. 



