194 DR. C. E. TILLET ON THE PETROLOGY OF [vol. lxxix, 



The problem at issue in regard to the Start Schists involves 

 "the consideration of them as 



(i) Pre-Cambrian or pre-Devonian rocks which have acquired their meta- 

 morphic features during- the Caledonian movements ; or 



(ii) Pre-Cambrian rocks which were already metamorphosed prior to the 

 Palffiozoic Era. 



The petrographic resemblance to the adjacent pre-serpentine 

 ■rocks of the Lizard suggests that the mica-schists and Green 

 ,Schists of the Start area and the Old Lizard Head Series (with 

 the associated hornblende-schists) have a common origin. 



Dr. J. S. Flett & Mr. J. B. Hill, in considering the question of 

 "the age of the Lizard group, and in reference to a possible Ordo- 

 vician date, state that the north-north-westerly strike of the 

 Lizard schists 



x is very strong evidence of their pre- Cambrian age, for neither in Britanny, 

 in South Wales, nor the South-East of Ireland, where extensive areas of these 

 rocks occur, has any reason yet been found to lead us to the belief that the late 

 Silurian, or Caledonian movements produced folds striking in that direction.' ' 



The magnitude and intensity of the folding during the Cale- 

 ■donian epoch do not appear to have been great in the South of 

 England, the regions of acute disturbance lying farther north. 

 The metamorphic effects, as revealed in the Ordovician rocks of 

 Cornwall, where a pre-Devonian folding has been recognized, are 

 (in those areas that are dissociated from thermal metamorphism) 

 >of no great magnitude. 



We are led to consider that the Start group of rocks should be 

 added to the pre-Cambrian, in which period the main meta- 

 morphism was already effected. The trend of the Armorican 

 folding, revealed in the Devonian rocks on the north, corresponds 

 •veiy closely to the west- and west-north-westerly trend of the 

 Start Schists. It appears not unlikely that the Armorican line 

 'represents a revival on an older trend-line of pre-Cambrian date, 

 extending from Britanny through the Channel tract, and including 

 "the isolated Start area. 



The post- Carboniferous movements which have set up slaty 

 cleavage in the Devonian rocks lying north of the Start area can 

 have effected but little mineralogical change in the Start rocks ; 

 but the movements may well have given rise to more complicated 

 mechanical structures in them, especially in the mica-schists. 



The question of the origin of the quartz and quartz-albite veins 

 in the mica-schists, and to a less extent in the Green Schists, is 

 • one of some interest. The abundance of quartz-veining in the 

 former might well be attributed to circulating solutions during 

 metamorphism. Quartz-veining, moreover, is not absent from the 

 Devonian rocks, and appears to coincide in direction with the 

 cleavage of these rocks. It is highly probable that the quartz- 

 veining in the mica-schists, as now observed, is the resultant of 

 circulating solutions during the folding movements which brought 



1 ' The Geology of the Lizard & Meneage ' Mem. Geol. Surv. 1912, p. 215. 



