THE GEOLOGICAL AGE OP CENTRAL AND WEST CORNWALL. 



29 



or during tlie extension of tlie granite, form a considerably more 

 elevated tract of country than the Fowey beds.* The deep valley 

 of the Fowey river seems to have been excavated for several 

 miles along the line of junction of the two series of beds. 



The Lower Silurian. 



The Ladock beds and the Fowey beds rest unconformably 

 upon a great series of schists and conglomerates, often of very 

 similar petrological and lithological appearance, and which have 

 a very regular S.W. strike. There is no doubt as to the age of 

 these rocks, they are known to be Lower Silurian, but their 

 extension in West Cornwall is very much greater than has been 

 hitherto supposed ; since they have been traced southward to St. 

 Keverne, and westward to Penzance. The great biilk of this 

 series consists of tolerably hard, thin-bedded rocks, mostly of a 

 dark grey or dark blue colour ; sometimes passing into an 

 inferior roofing slate, at others into soft sandy shales. 



Many of the beds are decidedly to be regarded as conglom- 

 erates, as may be well seen at the two Nare Heads, and on the 

 coast between Portholland and Port Luney.f In this latter 

 locality the total thickness of the conglomerate cannot be less 

 than 2000 feet. 



Besides these great beds of siliceous conglomerate, the beds 

 which possess the most distinctive character in this series are 

 certain moderately thick beds of quartzite, a small number of 

 thin lenticular masses of black limestone, many beds of highly 

 silicified slate usually known as greenstone, and until lately sup- 

 posed to be volcanic, and a number of thin beds of hornblende 

 schist and dark-green serpentine. 



Hitherto the siliceous conglomerates have yielded no fossils 



* There is no doubt whatever as to the unconformability of the two sets of 

 rocks — but the evidence of superposition is not so clear as could be wished. 



f Nearly the whole coast section from Portholland to Port Luney consists of 

 one great mass of conglomerate, which dips to the S.B. at an angle of about 30° 

 at an average. The distance is about fths of a mile, but as the beds are crossed 

 obliquely, the real thickness is pretty nearly what I have stated above. In this 

 conglomerate, as is usual with the old detrital rocks of Cornwall — the fine-grained 

 basis composes the greater part of the rock, pebbles and sub-angular masses of 

 pre-existent rocks somewhat sparsely scattered through it. Many of the frag- 

 ments closely resemble the rocks of the Lizard district. 



