190 THE GEOLOGICAL AGE OF CENTRAL AND "WEST CORNWALL. 



Instbusive Eocks. (Mica Trap). 



The remarkable group of veins of "mica-trap," " minette- 

 felsite" or " kersantite " whicli traverses the rocks of "West 

 Cornwall, was referred to in the former paper {2). .'J6), but has 

 never yet been described in detail. As we have devoted a large 

 amount of labour to the investigation of these veins, and as 

 they exhibit many points of special interest, we propose to 

 describe them here with some amount of detail. 



They traverse the stratified rocks in a great number of 

 places throughout a band of country extending from Eoscreage 

 Beacon, three miles south of the Helford Eiver, in a direction a 

 little to the N. of east, to Watergate Bay, a distance of nearly 30 

 miles, — the band affected being nowhere more than three miles 

 in width. The individual dykes for the most part have a course 

 about N.N.E., but they are often somewhat tortuous and 

 frequently split up into branches, some of which have for short 

 distances directions very different from that mentioned. 



So far as we are aware, these mica-traps — at any rate in a 

 characteristic form — occur in this narrow band of country, and 

 nowhere else in England, excepting only in the region to the 

 south of the Lake District of Cumberland. Until very recently 

 such rocks were altogether unknown in our country, but those 

 of the Cumbrian district were described to the G-eological 

 Society by Messrs. Bonney and Houghton in the year 1878.* 

 It will be seen hereafter that the Cornish rocks have many 

 characters and circumstances in common with the Cumbrian 

 rocks. 



We must remark, however, that some of the veins to be 

 described in the present paper have been previously referred to 

 in brief terms as " traps " and " elvans,"f but their connexion 

 with each other — their geographical limits, and their date as 

 eruptive rocks have, we believe, never before been the subject of 

 systematic enquiry. 



* Quart. Journ. Geol. Soc. 137, p. 165. 



t See De la Beche, Eep. Geoi., Corn. &c., p.p. 93, 94, where the " trap " of 

 Mawnau Cliffs is referred to ; see also A. K. Barnett " on the Elvan courses of 

 Coi-nwall," Rep. Miner's Assoc. Corn, and Dev. 1873; J. A . Phillips on " the 

 rooks of the mining districts of Cornwall, Qaart. Journ. Geol. Soc. i23 p. 337 • 

 and J. H. Collins, Trans. Roy. Geol. Soc. Corn. ix. p. 225. In this latter paper 

 the author described three type varieties of the Cornish "elvans," of which this 

 was one. 



