J. H. Collins — Cornish Serpentinous Rocks. 361 



detail ; but the descriptions not being generally known, as are those 

 of the Lizard, I refer to them here a little more in detail. 



A large proportion of the Extra-Lizard serpentinous masses seem 

 to have been at one time essentially hornblendic rocks. I do not 

 say originally hornblendic, but the production of hornblende has 

 preceded that of serpentine. Some, too, appear to have contained 

 considerable proportions of olivine, but this mineral does not, so far 

 as I am aware, exist in them in an unchanged state. 



Some writers draw a notable distinction between serpentine rocks 

 and those which are merely serpentinous. This is no doubt a con- 

 venient distinction in some instances ; yet it will be often difficult 

 to define the limits of this distinction, since there is no radical differ- 

 ence. On the one hand, there is no such thing as a rock-mass of 

 pure serpentine ; while, on the other hand, all serpentinous rocks, 

 whether approximately pure, porphyritic from the presence of un- 

 altered or only partially altered crystals of foreign matter, or con- 

 taminated by streaks and patches of foreign substance, are altered 

 rocks, and the serpentine present, whether much or little, is admittedly 

 an alteration product. To avoid naming such rock-masses serpentine, 

 as that of Duporth for instance, it has been proposed to class them 

 with picrite, but I cannot see that much is gained by this. Some 

 portions of this rock, it is true — from the presence originally of a 

 great many crystals of felspar, now mostly kaolinized — are indeed 

 highly aluminous ; but other parts contain very few such crystals, 

 and consequently the alumina sinks to an insignificant amount. 

 But, whatever the amount, the basis of the rock is plainly and dis- 

 tinctly serpentine, and even very pure serpentine ; and to call it a 

 picrite, apart from the objections to the use of this term as a rock- 

 name owing to its indefiniteness, seems to me not merely undesir- 

 able, but even somewhat misleading, if it be understood to beg 

 the question as to the nature of the original rock. Certainly there 

 is neither olivine nor diallage present now, and it is by no means 

 certain that the rock ever contained either. 1 



I will now proceed to give some brief particulars of the various 

 masses of rock outside the Lizard Peninsula, but within the mining 

 region of Cornwall and West Devon, which exhibit a high degree 

 of serpentinous change. 



1. Very an 2 (Gerrans Bay). — The serpentinous rock at Veryan is 

 a narrow band of dark green colour, which is associated with well- 

 marked gabbro similar to the rocks which form a great part of the 



1 I do not find picrite mentioned in my edition of Von Cotta. Dana says (p. 258), 

 Picryte from Moravia " consists half of chrysolite (olivine) along with felspar, 

 diallage, hornblende and magnetite." Rutley defines it (Study of Rocks, p. 265) as 

 " blackish -green crystalline rock with a compact black matrix containing porphyritic 

 crystals and grains of olivine. The matrix may consist of hornblende, diallage, or 

 biotite, associated with magnetite and calc-spar. The olivine constitutes nearly 

 half the bulk of the rock." Prof. Bonney regards it as a compound of olivine, 

 a small variable amount of felspar, such as anorthite or labradorite, enstatite or a 

 pyroxenic mineral with biotite. 



2 This district was surveyed by Mr. N. "Whitley, of Truro, in 1841, and described 

 in the Report of the Royal Institution of Cornwall. The serpentine was also 

 described by the Rev. Canon Rogers in the Trans. Royal Geol. Soc. of Cornwall, 

 vol. vi. p. 41, 1846. 



