Page:VCH Cornwall 1.djvu/90

 A HISTORY OF CORNWALL Culm series at Bude appears to have been deposited in tranquil water un- disturbed by strong currents. As the radiolarian chert of Mullion Island traced eastward into Goran gave a datum line for the Ordovician rocks of south Cornwall, so this northern chert acts as a datum for the lower Culm Measures of Cornwall, Devon and Somerset. 1 From Fire Beacon Point it is traced in surface stones and in a series of quarries and exposures from this point to the Tamar. A quarry 1 1 miles north of Launceston is marked on the 6-inch ordnance map ' Barracadoes Quarry (killas).' The beds in it however are not at all of the nature of killas, but consist mostly of light-tinted, banded, very hard siliceous rock with interbedded soft grey shaly beds, which are crowded with radiolaria and sponge spicules. Carzantic Quarry, 2 miles E.S.E. of Launceston, exposes a thickness of 50 feet of beds of compact chert without admixture of soft shaly beds, and throughout the series radiolaria can be distinguished, being in some beds thickly crowded together. If the formation of these cherts is mainly due to the silica separated from the sea-water by radio- laria, possibly also in part by sponges and diatoms, we must draw a heavy cheque on the bank of geological time to allow for the enormous interval necessary for the accumulation of such a thickness of rock by the gradual deposition of the skeletons of these microscopic organisms. Dr. Holl in his paper ' On the Older Rocks of South Devon and East Cornwall,' * gives the result of his careful survey of the Culm Measures of east Cornwall, and makes his line of junction between the Devonian and Carboniferous rocks cross the Tamar at Horsebridge near the horizon of Tavistock. The radiolarian character of the cherts in this district confirms Dr. H oil's view that they belong to the Culm series. Quarries near Landlake Cross, at Trenute, Trekenner Head, with out- lying patches at Painter's Cross, 4 miles north-west of Saltash, and a quarry 500 yards east of Pillaton church are all composed of radio- larian cherts, the southern ones being brought up by Lower Culm folds. Mr. S. R. Pattison, 3 in his paper on the Carboniferous system in Cornwall, says the plant remains in the broken cliffs close to the breakwater at Bude are found in greater numbers and better preserva- tion than elsewhere, although still in an extremely defective state as specimens. He records * Posidonice and Goniatites with plants at Truscott and St. Stephens, and Trilobites, Ortbides and Turbinolopsis at Under- wood and St. Stephens.' The rocks which enter into the composition of the Lizard pen- insula, as defined by the seaboard extending from Porthalla to Polur- rian Cove, are of a totally different character from the rock bands 1 ' On a well-marked Horizon of Radiolarian Rocks in the Lower Culm Measures of Devon Cornwall and Somerset,' by G. J. Hinde, F.R.S., and Howard Fox, Q.J.G.S. li. (1895), 609-67. 3 Trans. R. Geol. Sac. Corn. vi. 267-75. 42
 * Q.J.G.S. xxiv. (1868), 401-14.