TRANSACTIONS OF THE SECTIONS. 93 
term “Carboniferous slate” applied to this formation in Ireland by Sir R. Grif- 
fith. It is the lower limestone shale of Dr. Smith, as seen at Bristol and the 
Mendips. In this section, too, new bivalves occur in the basement-beds as at 
Caldy Island. 
Sundry other changes are observable when this section is compared with that on 
the east coast. The yellow conglomerate has disappeared ; and while red and grey 
conglomerate-beds are still plentiful on the north side of the bay, on the south 
side (a distance of barely a mile) scarcely a band of conglomerate can be traced in 
the first 80 or 90 feet, only 25 feet of which is of a red colour at all; the remainder 
consists of grey shale, yellow sandstone, and bands of limestone, which have only 
the faintest representatives on the north side. Grey shales, with plants, are mixed 
with these on both sides, 
The limestones are nodular, and contain crowds of Avicula Damnoniensis, Sow. : 
the characteristic shell of the Uppermost Devonian beds north of Barnstaple, North 
Devon (Rhynchonella laticosta, Phill.), occurs with it, together with species of 
Nucula, Axinus, Modiola, and Bellerophon, all of which are closely like, if not iden- 
tical with, Barnstaple species. The Serpuda-band before mentioned, at Caldy Island, 
occurs among these limestones, and at a somewhat greater distance below the base 
of the Carboniferous shales. 
By this remarkable change in the mineral character, accompanied by the intro- 
duction of a marine fauna, we are prepared for the still greater change in the Old 
Red sediments on crossing the Bristol Channel. The red tint is not, indeed, wholly 
lost between Ilfracombe and Barnstaple, but is confined to a narrow belt of rocks; 
and the Marwood beds, which are the equivalents of the uppermost red rocks of 
Pembrokeshire, are grey sandstones and olive shales, with calcareous bands, and 
with no red colour at all. They represent exactly the state of things (but on a 
much larger scale) above described on the south side of West Angle Bay. 
The Marwood Sandstones form a conspicuous group, ranging along a line five 
miles north of Barnstaple, and traceable east and west. They are well exhibited 
in the quarries at Sloly, on the Ilfracombe Road, at Marwood, Braunton, &c., and 
they form the headland of Baggy Point, where the best section occurs. 
In ascending order we have the— 
1. Red slates and sandstones of Morte Bay. 
2. A band of pale, nearly white slate, with a few bivalves. 
5. A thick series of greenish-grey grits, with bands of Cucullea and Avicula 
Damnoniensis in abundance, and with much olive shale, in which a new Lingulu 
occurs plentifully. (Marwood beds.) 
4, An alternating series of calcareous sandstones, grey shales with thin nodular 
bands of limestone, and grey cleaved slate, full of fossils, and many hundred feet 
thick ; Avicula Dammoniensis, Rhynchonella laticosta, &c., in all the lower part, and 
Strophalosia caperata and Spirifer Barumensis throughout. (Pilton Group.) 
This series (No. 4) is the upper part of the Pilton group of Professor Phillips, 
and its aspect im the grand coast-section is exactly that assumed by the Carbonite- 
rous shales which lie upon beds much resembling No. 3 (and with the same fossils} 
in the West Angle section. The author had previously suggested this explanation 
(see Address of Pres. Geol. Soe. 1855, p. xlviii). A more minute comparison of the 
two series convinced him that this identification (strongly advocated by Sir H. De 
la Beche) was erroneous, and that the Pilton group really represents a new series, 
including in an altered form the uppermost beds of the Old Red Sandstone*, toge- 
ther with certain beds at the very base of the Carboniferous shales. But in the 
main it is a new series, deposited in deepening water, while the Upper Old Red 
area was stationary (or nearly so) and close to shore, as evidenced by its plants. 
This series of beds has been described from the South of Ireland, by Professor Jukes 
and the author, under the term Coomhola Grits. It occupies there the same relative 
osition, overlying the true Old Red beds, and underlying the mass of the Carboni- 
erous slate. 
Some of its fossils are Carboniferous species ; but most of them, though strikingly 
similar, are not identical, and the presence of a common Devonian Trilobite through- 
out confirms the propriety of their first reference by Murchison and Sedgwick to the 
* Siluria, 2nd edit. p. 300, 
