STcomer Island, as seen from Wooltack Park, from a sketch by Mr. Francis Leach, 



CHAPTER XXX. 



PEMBROKESHIRE (continued). 



Silurian System. — Divided into Upper and Lower Silurian Rocks, with full deve- 

 lopments of the Caradoc and Llandeilo formations. — Cambrian System and 

 Slaty Rocks. — Trap Rocks, bedded and amorphous. — Divergent lines of strike 

 explained by eruptive ridges of trap. (PI. 35. figs. 1, 4, 6, 10, 11, and 12.) 



In the last chapter we have proceeded in regular descending order, from the surface 

 of the coal-measures, as being the youngest strata in Pembrokeshire, till we reached 

 those rocks which have been traced without interruption, from Shropshire to the western 

 borders of Caermarthenshire. 



The order in which they are arranged, and the organic remains which they contain, 

 clearly mark the age of the Silurian rocks in this district. Their aspect, however, and 

 their relative proportions of sandstone, limestone and shale, vary so much from those 

 in the strata selected as types in Shropshire, that it is not often practicable to make 

 those distinctions which enabled us elsewhere to subdivide the system into formations. 

 Thus, though certain gray rocks with fossils, occupy the place of the Ludlow formation, 

 yet they are, for the most part, unlike the rock of Ludlow, being harder and more siliceous • 

 whilst in the absence of any subordinate course of limestone, like that of Aymestry, we 

 cannot attempt to separate the formation into upper and lower portions. In like manner, 

 though the place of the Wenlock limestone is clearly manifested in Marloes Bay, it is rarely 

 to be known by its lithological structure, being for the most part hard and slaty ; though 



3 c 



