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 peri- 

 dotite. 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, horn- 

 blende 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 estab- 

 lished, 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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