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156
A TREATISE ON GEOLOGY.
CHAP. VI.
been found abundantly in many parts of Scotland, and rarely in Herefordshire and Breconshire.
We now pass the Severn, and find a great change. North Devon exhibits both the old red series and the true carboniferous series in an aspect much different from their northern types. The series stands thus:—
Pilton group.—A series of sandstones and shales, with sub-calcareous beds or nodules, very fossiliferous and much analogous to the lower carboniferous shale. Below are the
more truly Devonian equivalents of old red.
Morthoe group.—Fine grey, green or purple slaty beds, with sandstones and argillaceous shales. No fossils.
Ilfracombe group.—Argillaceous slates and limestones, corals, brachiopoda, plants.
Martinhoe group.—Red, brown, grey and claret coloured grits and slates. No fossils traced.
Linton group.—This is a thick mass of laminated grey grits and hard shales, partially affected by slaty cleavage. The fossils are not of many species, but are extremely frequent, and appear the more strikingly in this protoxidated mass, because the red groups above and below are devoid of organizations.
Foreland group.—Red and grey grits.
We may now proceed to South Devon. The groups here observed are not to be placed in strict mineral or structural affinity with the coeval series in the north. The best section is afforded in Plymouth Sound, but enormous contortions prevail.
Red sandstone group.—Consists of red sandstone, and schists alternating with grey and purple shales and schists. Polypiaria, crinoidea, brachiopoda, in the latter.
Grey schists and calcareous beds.—This is a very large and complicated group with trappean and ashy beds interposed irregularly. Polypiaria, crinoidea, brachiopoda.
The Plymouth limestone series.—A few partings of shale in this otherwise very solid coral rock, for such in a great degree it really is.
Purple slaty rocks,—still retaining their lamination. No
fossils.
By tracing these beds farther westward Silurian strata appear beneath them.
Organic Remains.—By uniting in one summary
