Page:VCH Cornwall 1.djvu/93

GEOLOGY

phism after the consolidation of the rocks from the original magma. On the other hand the foliation sometimes present in the gabbro is ascribed to movements when the rock was in a condition of imperfect fluidity, and not to pressure after the consolidation of the rock.

The cleavage of opinion regarding the relative ages of the serpentine and hornblende schist is equally well marked. On the one hand it is maintained that the hornblendic and granulitic groups were in their present condition when the rock which is now a serpentine was injected, the serpentine being the result of the alteration of some variety of peridotite. It is further contended that the granulitic group includes two types of rock, one acid, the other basic, the former being intrusive into the latter, and that as a result of this increase of temperature, or from some other cause, the temperature of the whole mass was so elevated as to permit of movements that gave rise to the finely stratified appearance of the two varieties.

On the other hand the opinion is held that the serpentine, hornblende schists, etc., form a banded complex of crystalline foliated rocks, the relative ages of which cannot be satisfactorily determined; but if there be any difference in age that the serpentine is probably the earlier of the two.

ADDENDUM

Since this article was written we have obtained important evidence bearing on the relations of the Devonian and Silurian formations of Cornwall. In the previous pages we referred to the fact that the conglomerates of Nare Point, Grampound, and Probus probably represented an unconformity dividing those formations. That inference has now been finally established. The Nare Point conglomerate is seen at Gillan Harbour in association with the Portscatho series, and the boulders of the former have been mainly derived, from the latter series. The fact, moreover, that the fragments were veined with quartz prior to such incorporation sufficiently indicates the magnitude of the break represented by this unconformity. The conglomerate is associated with sandstone and clay slate that are often slightly calcareous. The conglomeratic deposit is by no means continuous, so that the finer beds often represent the base of the formation, where they may be seen at Manaccan wrapping round the bosses of Silurian quartzite. The identity of this younger sequence of Manaccan with corresponding beds at Probus and Grampound has been further established, and as the latter in their northward extension pass into fossiliferous horizons of Devonian Age, this conglomeratic series represents the base of that formation. In the Meneage peninsula these rocks extend in a westerly belt from the coast between Gillan Creek and Nare Head. In that westerly extension they abut for some distance against the Lizard metamorphic rocks, the margin of which is otherwise in contact with the Silurian formation. The junction therefore between the Lizard rocks and the killas must represent a profound discordance.

The Portscatho series, therefore, and the associated Falmouth, Mylor and Veryan beds belong to the Silurian system; and as the Veryan beds appear to pass into the Caradoc quartzite, they may probably be relegated to the lower division of that system. The upper Silurian division is nevertheless represented, as determinate fossils have been recorded at Gorran Haven, Porthluney, and other localities.

The Devonian of Meneage may be regarded as a large outlier amongst the Silurian horizons. The main boundary between the two formations extends in an easterly direction from Porth Towan to Probus and Creed, it then passes to the north of Tregoney, from which it sweeps round to the south and thence follows an irregular course to the coast at St. Michael Caerhayes.

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