Chap. III.] LLANDEILO BOOKS IN PEMBROKESHIRE. 
53 
separate the one from the other, lithologically or stratigraphically, as it is 
to separate the black Alum-slate of Sweden (to be afterwards described) 
from the overlying Silurian rocks of that country. It is in these cliffs that 
the largest specimens of Paradoxides have been found by Messrs. Hicks 
and Salter. These, the lowest fossiliferous strata of the tract, graduate 
upwards into dark-coloured schists and slates, such as those observed on 
the west flank of the Stiper Stones, some of the best preserved Trilobites 
of the lower portion of the Llandeilo-flags having been obtained in this 
district. Among these are Ogygia peltata, very like 0. scutatrix (figured at 
p. 51), Trinucleus Gibbsii, and another species — together with Agnostus 
pisiformis, so abundant in North Wales. Again, there is the same passage 
upwards into black schists, with Trilobites and Graptolites, as at Abereiddy 
Bay, where the planes of slaty cleavage, as represented by the dark tint 
in the following woodcut, taken from the ' Silurian System,' coincide with 
the layers or beds containing the fossils. 
This Graptolite-schist is chiefly characterized by Didymograpsus Mur- 
chisonii, Beck (PI. I. f. 1), and Calymene duplicata (PI. III. f. 6). 
Lower Silurian Rocks in Abereiddy Bay. 
(From Sil. Syst. p. 400.) 
Coincidence of Slaty Cleavage and Bedding. 
To the south of these localities all the Silurian rocks are much obscured, 
the surface being to a great extent occupied by Old Eed Sandstone, Car- 
boniferous Limestone, aud Culm (see Map). 
In South Pembroke, the upper and chief portion of the Llandeilo de- 
posit is alone exposed in the cliffs at Musclewick Bay, where it contains 
Asaphus tyrannus and Trinuclei. The black- schists of this age are 
there insulated by a powerful dislocation, which, on their northern side, 
has placed them abruptly in contact with the Old Red Sandstone ; whilst 
towards Marloes Bay, or southwards, where younger Silurian rocks form 
the cliffs, the relations are obscured by a protrusion and overflow of 
igneous rocks, chiefly greenstone *, as in this diagram. 
Llandeilo Schists in Musclewick Bay. 
(From Sil. Syst. pi. 35. f. 11.) 
o b b * 
o. Old Red Sandstone. b. Llandeilo Schists. * Eruptive Rocks. 
* See Map. This coast and the dislocations of the strata will be subsequently described. 
