108 Mr. Hawkins on the Nomenclature of the Cornish Rocks. 



There are evidently numerous varieties of the Cornish 

 elvan, in some of which may be traced the transition from 

 porj)hyry into granite. The mass contains, moreover, in a 

 state of dispersion, many extraneous crystalUzed substances, 

 and is intersected by veuis of difierent minerals, which veins 

 are of contemporaneous formation. The circumstances under 

 which the elvan occurs, as we learn from the instructive re- 

 port of Mr. Carne, are in many respects singular, and, inas- 

 nuxchas they are connected with the natural history of our tin 

 and copper lodes, highly important. We have still, however, but 

 an imperfect knowledge of these, and I trust that those mem- 

 bers of our Society, who may have opportunities for collecting^ 

 farther information, will not neglect to avail themselves of 

 them. By way of hint to assist their inquiries, I beg leave to 

 mention, that in addition to the many extraneous substances 

 which have been observed in the elvan, may be placed the 

 tourmaline which I found in a quarry of this rock between 

 Grampound and St. Austle. I shall moreover Lake this op- 

 portunity of recording a very old observation of my own, which 

 throws some light, imperfect as it is, on the mode of its fox*- 

 mation. I allude to a smooth spherical mass of elvan, about 

 the size of a large cannon-ball, which I once found imbedded 

 in a solid block of the same rock. This ball of elvan appeared 

 to have acquired its shape by long attrition, before it became 

 inclosed in a fresh deposition of the same mass. 



I shall conclude with recommending a more accurate dis- 

 crimination of all the varieties of our granite, and an inquiry 

 uito their relative situation. In an attempt which I once 

 made to express by colours on Martyn's map, the different 

 sti'ata of the county, I perceived that the granite hills formed 

 several distinct and insulated groups of a circular form. Each 

 of these in my opinion deserves a particular examination, with 

 a view to the decision of a question of no small importance, 

 whether these insulated portions of granite are, strictl}^ speak- 

 ing, of homogeneous formation. That variety of granite, 

 which, besides the three constituent parts of the mass, in their 

 usual relative proportion, contains large perfect crystals of 

 felspar in a state of dispersion, appears to me to be peculiar to 

 the west of England, although even there not of general oc- 

 currence. The Dartmoor granite is of this kind ; likewise that 

 of St. Just, Sancreed, Sennen, St. Levan, Buryan, Paul, Ma- 

 dron, and Gulval. Should the remaining parishes of the 

 peninsula westward of Penzance be composed of it, we must 

 necessarily draw this inference, that the strata which constitute 

 the granitical formation in Devon and Cornwall, belong to 



two 



