" GREENSTONES " OP WESTERN CORNWALL. 169 



Cornwall extend from the northern shore of Porthleden Cove to a 

 quarter of a mile beyond the Levant Mines ; their total length is 

 about two miles, and their greatest width not above three hundred 

 yards. In many places, however, they form a mere border along 

 the coast line. 



These rocks, which are stratified, and have a general dip from 

 the granite, vary considerably in colour, since they graduate from a 

 bluish-grey through sundry shades of green to dark brown or black. 

 Slaty cleavage is often well defined, but in all cases in which it has 

 become partially obliterated it is rendered apparent by weathering, 

 and there is always a tendency to break into thin plates. 



They are generally lighter in colour than the slates bordering 

 Mount's Bay, and hornblende is not unfrequently more or less con- 

 centrated in laminae parallel with the cleavage. In some places, 

 and particularly in the cliff near Botallack, the rocks are traversed 

 by veins of hornblende, axinite, garnet, or magnetite ; garnets are 

 also sometimes plentifully imbedded throughout the rock. 



In some localities along this part of the coast the slaty beds 

 enclose a considerable amount of quartz in the form of layers fol- 

 lowing the lamination of the rock ; more rarely it forms distinct 

 veins crossing the lines of bedding. Wherever such an admixture 

 of quartz with slaty rocks is observed, the parallelism of the laminae 

 is interfered with, and the rock becomes more or less bent, cor- 

 rugated, or twisted; this contortion may perhaps to some extent 

 have been the result of crystallization*. 



Thin sections of these rocks are found to be composed of a colour- 

 less granular base, throughout which reticulated feathery horn- 

 blendic crystals are thickly disseminated, together with grains of 

 quartz, magnetite, sometimes minute garnets, and a few flakes of 

 brown mica. This mixture is occasionally traversed by acicular 

 hornblendic crystals, which are frequently arranged in stellate 

 groups of about T ji- inch in diameter. Sections cut perpendicularly 

 to the foliation show that the magnetite and hornblende are some- 

 times arranged in corrugated bands, and that the proportion of 

 transparent base is much larger than in the somewhat similar rocks 

 of the vicinity of Penzance. 



* I stated in a former paper (Quart. Jo urn. Geol. Soe. vol. xxxi. p. 341) 

 that it is not improbable that the repeated widenings which hare evidently 

 been experienced by fissures enclosing mineral veins, may sometimes have been 

 produced by mechanical forces resulting from ci'ystallization. As somewhat 

 supporting this hypothesis, it may be stated that, in the manufacture of " soda 

 crystals," at the chemical works of Messrs. Gaskell, Deacon & Co. at Widnes, 

 Lancashire, vessels made of thin boiler-plates have been substituted for the 

 ordinary crystallizers of cast iron. The result has been that a film of the 

 solution of carbonate of sodium found its way between the laps where the 

 plates are rivetted, and in crystallizing forced the sheets of metal apart. 

 To such an extent were the vessels damaged by this action, that they very shortly 

 required to have their joints reclosed by careful caulking. Since the pub- 

 lication of the paper above referred to I find that Volger attributes the forma- 

 tion of the fissures enclosing mineral veins to the force of crystallization, and 

 that the notion of vein-fissures being opened as crystallization advances is 

 defended by Griiner. See ' Chemical and Geological Essays,' by T. S. Hunt, 

 F.K.S. &c. p. 202. 



