part 2] THE METAMORPHOSED ROCKS OF THE START AREA. 195 



about rnetamorphism, and of solutions acting during the post- 

 Carboniferous activity, the evidence of which is their presence in 

 "the adjacent Devonian rocks. 



Quartz-albite- and albite-veins are not infrequently met with in 

 "the mica-schists, and Ussher showed that they are also present in 

 the Devonian rocks. These can be recognized at points north 

 of the boundary-line (for example, at Clannacombe, and at other 

 places as far as 2| miles north of the metamorphic boundary). 



The origin of these quartz-albite-veins is not free from 

 •obscurity : 



(i) They may be considered as having their source in the 

 epidote-albite-schists of the Start group, in which circulating 

 solutions have dissolved albite formed from plagioclase during 

 rnetamorphism, and with quaitz impregnating the adjacent mica- 

 schists as well as in the second period of movement penetrating 

 the Devonian rocks on the north. They are much more abundant 

 an the mica-schists than in the Green Schists, where they are rare, 

 and the more perfect schistosity of the mica-schist group is 

 doubtless the reason for this. 



(ii) They may be considered as having their source in the post- 

 Carboniferous intrusion of Dartmoor, and its apoph} r ses. Although 

 "the Dartmoor granite shows little sign of alkaline affinities, it is 

 nevertheless undoubted that the final products of granitic magma 

 are not infrequently sodie in character. Albite-pegmatites, and 

 albitites are known as dyke-rocks in association with normal 

 granites. 1 In Britain the albite-pegmatites of Leinster associated 

 with Caledonian crust-movements are examples. 



The most important evidence, however, that contradicts this 

 latter view is the absence of quartz-albite-veins in the granite of 

 Dartmoor, or in the Devonian sediments — other than those lying 

 immediately north of the Start boundary. The rocks of North 

 Devon have not yet been found to contain quartz-albite-veins, 2 

 although quartz-veining is not infrequent, nor do such appear to 

 be present in the sediments bounding the Dartmoor granite on the 

 south. 



We are led to the conclusion that the albite has its source in 

 the Green Schists, circulating solutions leading to a permeation of 

 the surrounding sediments. 



The analyses of the two petrographic types of Green Schist 

 occurring in the Start area show that these rocks are indistinguish- 

 able from normal basalts : the resemblance is complete. These 

 types were selected from rocks free from muscovite or abundant 

 quartz. 



There can be little doubt that the true Green Schists are of 

 igneous origin. Whether they are to be grouped as tuffs, lavas, 

 •or intrusive sills is a question of importance. Ussher saw fit to 



1 Such as those in association with the adamellites of South Australia ; 

 see Trans. Eoy. Soc. S. Austr. 1919 & 1920. 



2 Dr. J. W. Evans in litt. 



