KEMNANTS OF EXTINCT VOLCANOES. 455 



stances, viz., a gentle overland wind, and at very low water, so 

 there is not anything speculative put in to fill up in the section 

 of these rocks which I have drawn. 



The Looe and Swanpool bars are built up with quartz and 

 quartzite pebbles from the fissures referred to. The tufa is very 

 abundant at the western Nare, Belurian Cove and Asparagus 

 Island. In some places it undoubtedly obscures portions of the 

 vein quartz and the rocks, that contain them. 



Eespecting my research on the rocks of the Lizard, and the 

 views I have for years held concerning their volcanic origin, I 

 will simply say that the probable delay in the discovery of their 

 true origin was caused by the non-recognition of the rapidity — 

 geologically speaking — of the change a greyish olivine makes 

 when coming into contact with certain lime felspars, which are 

 very abundant in the Lizard district. 



The result of the change referred to is a transformation of 

 these two minerals into one, known as Anthophyllite, which is 

 a secondary hornblende ; geologists, until very recently, were not 

 prepared to receive such a change as genuine, but a view of my 

 slides from Coverack and St. Keverne must, I think, convince 

 the most sceptical of the fact. 



I have found similar changes in a coarse greenstone at 

 St. Mewan, near St. Austell, and in the felspathic tuff at the 

 Nare head, Veryan. 



The brownish and yellowish olivine when found associated 

 with enstatite, or bronzite under favourable conditions, slowly 

 changes into serpentine, but the tufa of the Western Nare is of 

 a much earlier date, and is undoubtedly Silurian. 



In a slide of a somewhat modern gabbro from Coverack, on 

 the table, there is to be seen the end of an olivine crystal in 

 contact with augite ; at that point there is no perceptible change, 

 but at the other side where it is in contact with labradorite, 

 (soda-lime felspar) the change is far advanced, and beautiful 

 zones and needles of anthophyllite have been formed. To many 

 this would be sufficient proof of the true volcanic origin of the 

 rock from which this section was taken ; olivine being a well 

 known volcanic production. A great portion of the so-called 

 hornblendic schist of the Lizard is undoubtedly derived from 

 the passage of olivine into anthophyllite. 



