REMNAJS^TS OF EXTINCT VOLCANOES. 453 



The next layer is very much in excess of the former and is 

 an extensive mass of conglomerate, priacipally composed of 

 broken gabbro or felspathic tuff. To the north of the neck we 

 have what appears from surface view, to be a dyke of matter 

 very far advanced into serpentine, with but few traces of the 

 original mineral forms distinguishable. Between the serpentine 

 and the neck of gabbro stands an almost unaltered portion of 

 Silurian slate, which is an evidence that con'tact metamorphism 

 has had but little effect there. If we go a little to the north of 

 the serpentine, on the old stratified rocks we find ourselves 

 immediately confronted with immense blocks of quartz and 

 quartzites many tons in weight, and so numerous that for some 

 distance the land is not worth cultivation ; these quartz rocks 

 were probably the product of the first hot water which issued 

 from the heated and already expanded and fissured region. This 

 water or fluid containing a high per-centage of silica when it 

 arrived at a cooler and more tranquil portion of the fissures, 

 crystallized into the form in which we find it. Since this crystal- 

 ization took place there has been great denudation of the 

 surrounding rocks, and now the quartz blocks stand forth in 

 bold relief or have fallen and rolled over in great profusion 

 along the hillside^' 



Blocks of quartzites are also abundant yet retaining visible 

 bands or lines of its former stratification, and also containing 

 some perfect casts of Lower Silurian fossils. 



A view of the cliffs from the point marked on the map, 

 Penarin Point, will be found to be fissured and filled with 

 quartz, and will give the concluding evidence of the nature and 

 character of this very interesting region. There is no mining 

 district in Cornwall where quartz is so abundant, and it is only 

 approached by the gold mining regions of California. I should 

 not consider it a very forlorn hope to seek for gold in the drifts 

 of these quartz rocks. 



I now propose to refer to the land south of the Helford 

 Eiver, or that portion of Cornwall known in many a geological 

 debate as the Meneage Peninsula. This has long been a hot bed 



* See Ordnance Survey Map, County of Cornwall (Western Division,) Sheet 

 LXV, Nos., 1495, U'Jd, 1500. 



