Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1054

924 not true granite, but only an altered sedimentary rock, we have, so far as I know, no clue.

The sedimentary rocks of the Lizard peninsula are probably about Lower-Devonian age. The great granite masses further north are probably late Carboniferous, or at any rate pre-Triassic. Doleritic outbreaks, such as might have been fed by the great gabbro massif, the form of which seems to suggest the probability that it was once deep below a volcanic cone, occur both of Devonian and Carboniferous age; and the latest known igneous action in "West Wales" is recorded by the Triassic basalts in and about Exeter. We may then venture to conjecture that the intrusion of the lherzolite was of later Devonian age, and that both the gabbro and the granite may belong to some part of that immense period when the Coal-measures of Central and Northern England were being deposited and afterwards denuded before the "New Red" series was formed. Possibly the dark traps may record the slight sporadic igneous action indicated by the Exeter basalts, and so be of Triassic age. The aspect, however, of some of these seems indeed to suggest that they solidified at no great distance from the surface, and so under conditions not very different from those at present existing. Hence it is possible that these veins, together with the phonolite of the Wolf Rock, are remains of a yet later period, the Miocene, during which volcanic action was so rife in Scotland, Central France, and Germany.

These rocks may be roughly divided into four principal groups (without enumerating others of less importance), viz.:—

Features in common.—In all specimens of the above rocks examined by me, alkalies are extremely scarce, if not altogether absent in most cases. They are all basic, rarely containing as much as 50 per cent, of silica.

Differences.—Chemically they divide themselves into two groups:—Group A (poor in lime and alumina) includes the serpentines; Group B (rich in lime and alumina) includes the "greenstones," hornblende schists and gabbros—rocks which, however much they may differ in appearance, have considerable resemblance in their fundamental constitution.