﻿Vol. 64.] OKIGIN- OF THE PILLOW-LAVA IN CORNWALL. 265 



Pillow-lavas have been described from many parts of the world, 

 and the isolation of the individual pillows has been commented on 

 by various writers. Banded vesicular structure has also been 

 referred to as occurring towards their periphery ; more rarely it 

 has been noticed as increasing towards the centre. The origin 

 of the pillow-structure has been much discussed, but the general 

 trend of opinion is in favour of the view that the lava-streams 

 flowed into or under water. Pillow-lavas differ so greatly in 

 different localities in various parts of the world, that it seems very 

 doubtful whether all can have originated in the same way ; they 

 are also widely distributed in time. The pillows described in this 

 paper are of a peculiar and apparently unusual type. 



Description of the Occurrence. 



The area around Port Isaac has been mapped on the 6-inch 

 scale by us for the Geological Survey during the past two seasons, 

 with the result that the complicated outcrops of lava prove to 

 belong almost entirely to a single sheet. This sheet is interstratified 

 near the top of the grey slates which form the lower part of the 

 Upper Devonian strata of this region. A little lava of similar 

 character occurs at a somewhat higher stage, in purple and green 

 slates ; but our description will be confined to the more massive 

 sheet, probably 200 feet or more thick, seen in the cliffs on each 

 side of Port-Quin Bay, and in inland quarries in the parishes of 

 St. Minver and St. Endellion. 



The best places to study these lavas are the bold cliffs of Pentire 

 and Kellan Head ; for, though much of the cliff-face is inaccessible, 

 plenty of good exposures can be found without dangerous climbing. 

 Clear inland sections may be seen at various places, some of them 

 being close to the high road. Por instance, the Church-Hill Quarry, 

 from which the road-metal is obtained, is just outside Port Isaac. 

 The quarry by the roadside shows the pillows obscurely ; but 

 immediately above, though invisible from the road, is a much finer 

 section in a newer quarry. Very fine exposures are also seen 

 inland in a deep quarry a quarter of a mile north of Trelights, and 

 in shallower pits just west of Trelights. 



The cliff at Pentire shows a mass of pillow-lava to a height of 

 258 feet; the Kellan-Head section is about 200 feet high. In 

 neither case, however, is it possible to say whether the total 

 thickness may not be much greater, or whether, on the other hand, 

 there may not be some duplication by overthrusting. We can be 

 certain, however, that great masses are quite free from shear-planes. 

 So far as we have been able to ascertain, this is the maximum 

 visible development of the sheet, which would suggest that the vent 

 must be sought for more to the north, and under the sea. 



A series of photographs of the sections has been taken for 

 the Geological Survey by Mr. T. C. Hall. The two photographs 

 selected for reproduction show the characteristic features of the 



