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An Essay in Geographical Evolution

By Arthur W. Clayden - 1906.

Sample text :-
' They have long known as the Coddon Hill beds, from a conspicuous hill about a mile south of Barnstaple, and forming the southern side of the valley in which the black limestones of Venn were formerly worked. In a quarry, on the north-west of the hill, the beds are admirably shown. There are some of them almost as hard as porcelain, and of various dark or mottled shades of colour, but some are streaked with white, like a porcelainized flint, while some of the intervening beds are a powdery pale grey shale which can be crumbled between the fingers.

Precisely similar beds form a line of bold hills stretching eastwards to near Ashbrittle. Here they are lost, and are apparently represented by the chert bands so intimately associated with the limestones of Westleigh and Holcombe Rogus.

The same beds crop out above the southern limestones and have been traced from the Cornish coast past Laun-ceston, round the northern edge of Dartmoor, to the neighbourhood of Chudleigh.

When, in 1893, Dr. Teall and Mr. Howard Fox discovered the Radiolarian origin of the chert at Mullion Island, the idea suggested itself to many that the Coddon Hill and other chert beds might possibly have been formed in a similar way, so that when in 1895 an exhaustive paper on these beds was read before the Geological Society by Mr. Fox and Dr. G. J. Hinde, Devonshire geologists were not surprised to learn that it was proved that these basement beds of the Culm repeated the deep sea phenomenon of the earlier date.

We are now in a position to attempt a reconstruction. We may regard the massive limestones of the Midlands as the solidified ooze of a moderately deep sea. It will be remembered that while there was land where the Carboniferous Limestone was subsequently deposited there was open sea over Devon. If now, as we have pointed out was possibly the case, the rather rapid subsidence was general over a wide district, a lowering enough to convert the land into a deep sea would necessarily convert the neighbouring seas into very deep ones. '

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