500 MR. B. HOBSOKT ON THE BASALTS AND 



Cornwall, 55 N.W.), a few yards north of the Cawsand rock, of an 

 outlier of New Eed breccia.^ 



One of the most remarkable points about the Cawsand rock is the 

 presence everywhere in it of macroscopically evident flow-structure. 

 The Withnoe rock, described by Mr. Worth, is exposed in a small 

 quarry on the east side of Cliff Lane, leading southwards from 

 "Withnoe to the cliffs of Whitesand Bay (6-inch map, Cornwall, 

 54 N.E.). It certainly resembles the Cawsand rock macroscopically, 

 and shows the same evident flow-structure, which is vertical or 

 highly inclined. The same Withnoe rock is much better exposed 

 in a quarry 450 yards S. 35° W. of the five cross-roads at Withnoe, 

 a few yards north of the " Old Quarry " on the 6-inch map, on the 

 N.E. side of the military road running along the cliffs. Here it is 

 seen to be intrusive in the Devonian slates, but the most interesting 

 point is the occurrence in this quarry of four dykes (perhaps not 

 all separate) from 1 to 4 feet wide, of apparently quartz-porphyry 

 intrusive in the Withnoe rock, cutting across the almost vertical 

 flow-structure. The quartz-porphyry is pinkish grey to purplish, 

 with phenocrysts of sanidine-like felspar (measuring as much as 

 1 X J inch), kaolinized felspar, dark mica, and quartz in a compact 

 groundmass. 



y. Othee Alleged Inteusive Dykes ts the Lavas. 



Mr. E. N. Worth remarks with regard to the ' felspathic traps,' 

 " They are traversed at so many points by felsitic dykes that this 

 association has a constant and not a merely casual character. 

 Mr. Yicary noticed these dykes and their strong resemblance to 

 elvan courses, though, as ' a matter of convenience,' calling them 

 sandstone." ^ 



As a matter of fact, I believe that there are no felsitic dykes 

 traversing the undoubtedly Permian (or Triassic) ' felspathic traps,' 

 but I have not examined every exposure minutely enough to assert 

 positively that there are none. The so-called felsitic dykes are 

 nothing but exceedingly calcareous sandstone- veins, or veins of 

 calcite stained red. Such veins are of common occurrence in rocks 

 erupted sub-aqueously and overlain by sandstones.^ 



I observed these veins at Posbury, Eaddon Court, Orchard Copse 

 at Dunchideock, and at Pocombe. They vary from pink to brick- 

 red in colour, and from a fraction of an inch to 3 inches in breadth. 

 In some cases compact-looking, in others, e. g. at Posbury (No. 929 d), 

 they show sand grains when examined with a lens. These grains 



1 W. Pengelly, Trans. Devon. Assoc, vol. ix. (1877) p. 418 ; E. N. Worth, 

 Trans. Eoy. Geol. Soc. Cornw. 1886, p. 231. 



^ 'The Igneous Constituents of the Triassic Breccias,' &c., Quart. Jouru. 

 Geol. See. vol. xlvi. (1890) p. 80. 



^ See A. Geikie, Explanation of Sheet 14 (p. 13) and Sheet 13 (par. 5), Geol. 

 Surv. Scotland ; also in Geol. Mag. for 1866, p. 244, Anniversary Address 

 to Geol. Soc. 1892, p. 82 (p. 58 of separate copies), ' Text-book of Geol.' 1885, 

 p. 549 ; and R. D. Irving, ' Copper-bearing Rocks of Lake Superior,' U. S. 

 Geol. Survey Monograph, 1883, pp. 137-140. 



