456 REMNANTS OF EXTINCT VOLCANOES. 



There lias not been any evidence to show whether the 

 St. Keverne gabbros were ejected into air or water, but there is to 

 be found on Crowsa Downs, resting on these gabbros, a great 

 accumulation of quartz and quartzite pebbles, corresponding 

 with those at Swanpool and Looe Bars, but there has not yet 

 been discovered any organic remains, which would show the age 

 of their deposition. 



The Wolf Eock. 



This rock, situated about 9 miles south-west of the Land's 

 End, and about 22 miles west of the Lizard point, is surrounded 

 with from 30 to 40 (engineer's measurement) fathoms of water, 

 is about 175 feet in length, and about 150 feet in width, and 

 rises about 17 feet above low water; during the spring tides it 

 is covered with water to the depth of about 2 feet. On it is 

 erected a lighthouse, which has proved a great blessing to 

 mariners. The rocks excavated for laying the foundations of 

 the lighthouse have thrown a great deal of light on the geology 

 of the south-west coast of Cornwall, if not also on the tradition 

 of the submerged land of the vicinity ; but it is their composition 

 and relative age that principally interest the geologist. 



A description of the Wolf rock by Mr. Allport will, I think, 

 be found interesting. He writes : — "Examined by the eye, or 

 a small pocket lens, the rock is seen to consist of a yellowish 

 grey compact base in Avhich crystals of clear glassy felspar are 

 embedded, they exhibit no strise, but their fracture is sharp and 

 splintery." After a long microscopic description he arrives at 

 the conclusion which is especially worthy of remark, that "this 

 grey dust occurs in precisely the same way in the nephoKne of 

 the basaltic phonolite of Tertiary age, and from widely separated 

 localities." The Wolf Eock may therefore be considered con- 

 temporaneous with the Coverack gabbros. This is very conclusive 

 evidence that south-west Cornwall contains the Basal wrecks 

 and remnants of extinct volcanoes, and when, further, Messrs. 

 Allport and Judd, class these latter as Tertiary ejectments, 

 they confirm the views I have held for many years on them. 



It is interesting tracing their connection with other well- 

 known basaltic wrecks and elevations of land along the coast of 

 England and Wales. The Isle of Man, where the Pleistocene 



