164 THE GEOLOGICAL AGE OF CENTRAL AND TTEST CORNWALL. 



and " dimstones " of the eastern part of the county, which are 

 on all hands admitted to be Devonian.* 



As a matter of fact, until the communication of Mr. 

 Somervail's paper to the Eoyal Institution of Cornwall — in 

 which he expresses the opinion that these rocks are "in reality 

 about the lowest in the county" (see his paper. Jour. R.I.C., 

 Vol. VII, Ft. 4, f. 268y — we were not aware that anyone had 

 doubted the Devonian age of the rocks in Cjuestion. 



Referring to an outlying patch of rocks to the south of the 

 Helford River, believed to be of the same age as the Ladock 

 beds, the following was written in the former j)aper (p. 23) " The 

 area in questionf is composed chiefly of schistose rocks, but it 

 includes a remarkable bed of very coarse conglomerate, which 

 stretches westward and a little inland from the Nare Point 

 towards Trelowarren .... Some of the included masses in this 

 conglomerate weigh several tons, and are themselves portions of 

 a still older conglomerate." By following this bed at low water 

 along the shore westward towards Mushing, it may be seen 

 rising gradually so as to form low cliffs, the strata dipping very 

 gently to the southward. At several points the bed may be 

 seen to rest unconformably upon highly inclined slates, whose 

 strike is N.N.E. A conglomerate — which we believe to be the 

 same — appears oh the eastern side of the Point, and may be 

 traced southward as far as Nelly's Cove. 



*"l]irough the kindness of Mr. J. H. Collins I have been enabled to examine 

 fonr specimens of Cornish grit, namely, one from St Allen, four miles north of 

 Truro, two from Ladock, five miles further east, and one from Perranzabiiloe in 

 the Bristol Channel. Hand specimens of all these rocks closely resemble one 

 another, excepting that those from Ladock enclose numerous angular fragments 

 of a greenish slate, which the others do not, and one of them contains a number 



of rounded quartzose and other grains ^-inch in diameter Tn the rock from 



Ladock, which contains small rounded grains of quartz, felspar, and other 

 material, these bodies are sparsely disseminated throughout the mass of the 

 normal grit ; and a microscopical examination shows that some of them are 

 fragments of volcanic rocks closely akin to the " greenstones " and " dunstones " 

 of many parts of Cornwall, but -rthich have often become so altered as to be 

 recognizable only by their felspars and general structure." — " On the Consfitu- 

 tion and History of Grits and Sandstones," by J. A. Phillips, F.R.S., 

 Quart. Jour. Geol. Sac, February, 1881, p. 10, and also p. 25, and PI. 1, fig. 2. 



t This area was omitted from the map by a mistake of the engraver. A 

 sketch map of the district is now given on an enlarged scale. 



