﻿264 MR. C. REID AKD MR. H. DEWEY ON THE [May I908, 



13. The Origin of the Pillow-lava near Port Isaac in Cornwall. 

 By Clement Eeid, P.E.S., F.L.S., F.G.S., and Henry Dewey, 

 P.G.S. (Communicated by permission of the Director of 

 H.M. Geological Survey. Eead January 22nd, 1908.) 



[Plates XXVII & XXVIII.] 



The volcanic mass which we are about to describe occupies a 

 definite horizon in the Upper Devonian, in the part of North 

 Cornwall lying between Padstow and Bodmin Moor. We propose 

 to confine our remarks to this limited area ; for the point with 

 which we deal is merely the mode of origin of this particular 

 eruptive rock. Our views may, or may not, be applicable to other 

 pillow-lavas ; but the subject is so beset with difficulties, that we 

 prefer to confine our attention to a limited region, where cliff- 

 sections are exceptionally fine and inland quarries are also clear. 

 We will not deal with the question of correlation, even with neigh- 

 bouring areas. 



Previous Literature. 



The volcanic rocks near Port Isaac were noticed by De la Beche 

 in 1839.^ He mentions the vesicular character of some of them, 

 and suggests that they were contemporaneous with the slates in 

 which they occur. He does not allude to their peculiar structure. 



In 1848 iSTicholas Whitley noticed and figured the peculiar 

 concentric structure, 



'as if it had rolled down a declivity and become partially cooled during its 

 progress, and then consolidated into the rock which it now constitutes ; in 

 fact, much like the ends of bales of cloth piled one on another.' 



He speaks of the centre of each circle being generally composed of 

 a nodule of crystallized gypsum, and compares this structure with 

 that of some of the Yesuvian lavas." 



The petrological characters of these rocks were described in 1878 

 by J. A. Phillips,^ who gave an analysis of a specimen in which 

 the amygdules were filled with calcite. Only a small portion of 

 this calcite, however, can have belonged to the original rock, which 

 was once highly vesicular, though now solid through the infilling of 

 the amygdules. 



In 1902 Mr. Howard Pox described ' Some Coast-Sections in 

 the Parish of St. Minver.' ^ In this paper reference is made to the 

 spheroidal structure which is very frequent throughout aU this 

 range of cliff, and one of the spheroids is figured. 



^ 'Eeport on the Geology of Cornwall, &c.' pp. 88-89. 



^ ' On the Remains of Ancient Volcanoes on the North Coast of Cornwall, in 

 the Parish of St. Minver, &c.' 30th Ann. Rep. Roy. Inst. Cornwall, App. yi, 

 p. 62 (1848-49). 



^ Quart. Journ. Geol. Soc. vol. xxxiv, p. 483. 



* Trans. Roy. Geol. Soc. Cornwall, vol. xii, pt. riii, p. 670. 



