Geological Society. 563 
curate outline expressed in Mr. De la Beche’s map*, the author re- 
marks the existence of a double trough of the lower limestone shale 
overlying the old red sandstone in East Angle Bay ; and he particu- 
larly adverts to the peculiar mineral character of these beds in Pem- 
brokeshire in containing yellow and light coloured sandstones alter- 
nating with shale. The fossils of this lowest member of the carboni- 
ferous system are numerous, many having been furnished by the Earl 
of Cawdor ; and as far as they have been yet examined they appear to 
differ specifically from all the fossils of the inferior systems. The coal 
measures and mountain limestone of Pembroke are singularly subject 
to great faults ; one of the most remarkable of which occurs between 
Johnston and Haverfordwest, where the carboniferous limestone is 
thrown into a position by which it appears to overlay the coal. 
Old Red Sandstone.—The upper strata of this great formation pass 
upwards in many places into the shale and sandstone of the carboni- 
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 typesin 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 Ludlow, 
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 
* The survey of the county was much facilitated by the possession of Mr, 
De la Beche’s map of South Pembroke, which, though differing in some 
points from that completed by the author, is mentioned by him as a work of 
great merit for the period of its publication. 
+ See Lond. & Edinb. Phil. a ph vii. p. 46, Silurian System. 
3K 2 
