802 REPORT—1890. 
always made with extreme caution and great ability, in some cases, when studied 
by transferring the information to a map, prove to be contradictory. Although 
much involved, De la Beche’s descriptions of Cornwall seem to furnish some clue: 
to the structure of that county, when interpreted by a comparison with the known 
regions of North and South Devon. 
The following classification of the Cornish rocks is arrived at by an exhaustive 
study of De la Beche’s Report, and by some years of hard work in the Devonian 
rocks of North and South Devon, supplemented by careful observation of the coast 
from Plymouth to Looe, and by traverses in the Launceston, Petherwin, and Tavi-- 
stock country. These materials are sufficient to justify the classification as a sug- 
gestive and tentative one ; beyond this nothing is attempted. 
Upper Devonian. 
Tintagel and Petherwin Series with contemporaneous Volcanic Rocks.—The- 
major part of the series consists of grey slates, but red slates (St. Kew and St. 
Minver, St. German’s and Mutley, &c.) seem to predominate in the lower beds. 
Extent :—Pentire Point to St. Tudy and Eeloshayle, and round the Camelford 
granite, between Tavistock and Plymouth. 
Correlations :—With Entomis and Goniatite slates of South Devon. In the 
upper horizons, also with Livaton and Druid beds; with Pilton beds, Pickwell 
Down slaty horizons, and (?) Morte slates. 
Middle Devonian. 
Grey slates, with occasional limestone bands. 
Extent :—From Permizen Bay, toward St. Tudy, and perhaps on north of St. 
Breock Down eastward to Warleggon, Mount Edgecumbe, Landulph Promontory 
on the Tamar, and probably elsewhere in the neighbourhood. 
Correlations :—With slates between Plymouth and Totnes; with Ilfracombe 
series. 
Lower Devonian. 
Upper Coblenzian.—St. Breock Down and Bocoanoe arenaceous beds, Pickle-- 
combe and Maker grits. 
Correlations :—With Hangman series of North Devon; with Staddon and 
Cockington grits. 
Lower Coblenzian.—Mawgan slates, Tregantle limestone, &c. (?) Newauay 
slates. 
Correlations :—With Lynton beds of North Devon, Meadfoot beds of South 
Devon. 
Hunsruckien (?)—Variegated slates of St. Austell, extending from Talland, by 
St. Blazey to St. Stephen's, and from Tregoss Moor to Watergate Bay. 
Correlations :— With Dartmouth slates ; (?) with Foreland grits. 
Upper Gedinnien.—Looe Beds. Pencarrow and Gribbin Head, at the Black 
Head, and thence to the West coast near St. Cubert. If the Newquay slates do: 
not belong to this horizon, the red slates of Watergate Bay would be in an anti- 
cline, and be represented in the area between Oubert and Newquay by a continua-- 
tion of the St. Austell beds west from St. Stephen’s. 
Lower Gedinnien.— Grampound and Newlyn Down arenaceous rocks; possibly 
the base of the Devonian beds, and separating them from Lower Silurian rocks. 
extending to the south of the latitude of Grampound. 
The extension of the Grampound horizon south of Newlyn Down is inferred. 
by De la Beche, but it is not noticed in his section from Newdowns Head to the 
. Lizard (Plate II., Fig. 4). De la Beche correlated the Gorran limestones with 
those of the Looe beds, and regarded the red slates of Falmouth as a lower horizon. 
than the St. Austell band. 
The grouping south of Grampound may be inverted ; in which case (1) slates, 
with occasional contemporaneous volcanic rocks of Penzance, Gwinear, perhaps: 
Feock, might be uppermost, and in descending sequence therefrom (2) Falmouth 
slates, (3) Mevagissey slates, (4) Gorran, Veryan, Nare Head, and Porthalla beds. 
