Geological Structure of W. Cornwall. 



71 



For facility of reference it will be necessary to examine the 

 following sheets of the new series Greological Survey map of England 

 and "Wales, as it is obviously impossible to reproduce such a map in 

 this Magazine. 



Key-map to sheets of Geological Survey Map. 



This map agrees so closely in mapping with the results of our own 

 twenty years observations and is so confirmatory of our views that 

 Ave may well refer to it. The one serious error is the line of 

 'conglomerate' dividing the ' Grampound Grit' and the ' Porth- 

 scatho Beds ' almost at right angles to their strike on Sheet 353. 

 But as those two deposits are one and the same, this obvious error 

 need not detain us. 



At the foot of Sheet 353 is a section from Porthluney to the 

 Dodman, and this section will suffice precisely for our purpose. 

 We accept it, for it corresponds with our own observations, although 

 we place our own interpretation upon it. It will be noted that there 

 are several exposures of quartzite. These are the quartzites that 

 have yielded Ordovician fossils at Gorran, Diamond Point (Porthluney), 

 and Carne, and upon the age of which every observer is agreed. The 

 bands are definite although broken up, pushed about, and dis- 

 membered, and can be traced from Gorran through Porthluney to 

 Carne and are seen again westwards at Nare Point (Helford River) 

 and thence across the country to Cury, north-east of Mullion. This 

 quartzite is the oldest stratified deposit in Cornwall, except perhaps 

 the Dodman phyllites and the Lizard rocks, Avith which Ave are 

 acquainted, and we have never seen the bed upon which it rests, 

 unless the peculiar greenish-black quartzite containing Orthis calli- 

 grmnma (Brit. Mus. and Mus. Pract. GeoL), occasionally seen as 

 boulders in and near Catasuent Cove, may represent a lower bed. 

 Now let us interpret this section. To the north, at a point marked 

 Porthluney, is a little canal cut to drain the Carhayes Lake. A low 

 cliff borders the eastern side of the canal composed entirely of 

 Porthscatho Beds, though we believe the first few yards of the 

 northern end show Yeryan Beds at the base of the cliff. Passing 

 out of the canal and proceeding south along the beach, we encounter 

 the coarse ' basal conglomerate ' of the Porthscatho Beds, followed 

 by a mass of igneous rock (marked D) which has broken through and 

 disturbed the beds. At this point the interest begins, for we find 

 undoubted Veryan Beds beyond the igneous intrusion, containing 

 lenticles of fossiliferous Silurian rocks and continuing unaltered 

 south to the further horn of Catasuent Cove. lieyond Catasuent 

 Cove the coast for three-quarters of a mile is inaccessible to us, 



