part 2] THE METAMORPHOSED ROCKS OF THE START AREA. 181 



hydrated iron oxide, but the mineralogical constitution is not 

 affected. These rocks are, as a rule, distinctly schistose, though 

 more massive types are not uncommon. They show, in many cases, 

 alternations of colour from yellowish green to deep green parallel 

 to the schistosity, and this in some respects simulates a bedded 

 structure. In thin sections this structure may be revealed by 

 successive bands showing variable mineralogical composition, such 

 as narrow layers rich in albite adjoining layers in which chlorite 

 or hornblende is more abundant. On weathered surfaces this 

 pseudostratal structure is accentuated by the protrusion of small 

 crystals of the more resistant albite. Furthermore, the Green 

 Schists have not infrequently been penetrated along the planes of 

 schistosity by quartz solutions, but on a scale far from commen- 

 surate with that developed in the neighbouring mica-schists. The 

 resistant weathering of these narrow films of quartz tends further 

 to accentuate this structure. 



Many of the schists are characterized by a pitted or cavernous 

 structure, and these hollows can be shown to represent weathered- 

 out grains of calcite, or quartz of the quartz-veining. 



Among other macroscopic structures developed in the Green 

 Schists, we may note the presence of nodular masses largely con- 

 sisting of yellow-green epidote. Such masses, varying in size from 

 half an inch to 6 inches, occur in the Green Schists in various parts 

 of their exposure. Some of the best examples can be studied in 

 the coast-sections between the Bull and Limebury Point, at the 

 southern end of the Salcombe estuary, and also in the Prawle 

 mass of the Green Schists at Hamstone Cove, and the neigh- 

 bourhood. It is in every way probable that these represent 

 amygdales, and are the metamorphosed equivalents of infilled 

 vesicles in an original volcanic rock (see p. 187). 



I have already referred to the presence of quartz- veinlets in the 

 Green Schists. These have the same origin as those developed in 

 the associated mica-schists. In the latter rocks, however, they 

 are much more abundant than in the Green Schists. This 

 contrast can nowhere be better exemplified than in the narrow 

 bands of mica-schist associated with the Green Schists at their 

 lower and upper junctions. There can be no doubt that this 

 contrast is due to the fact that the invading solutions have selec- 

 tively penetrated the mica-schists, on account of their more 

 perfectly developed schistosity. Quartz-albite-veins must be quite 

 exceptional in the Green Schists themselves. 



Petrographically, the Green Schists can be divided into two 

 distinct types: — (i) chlorite-epidote-albite-schists ; and (ii) horn- 

 blende-epidote-albite-schists. There are gradations between these 

 types, in which chlorite and hornblende are almost equally developed. 

 Nor can we separate these two dominant types with regard to 

 any zonal distribution, for both are to be recognized in all the areas 

 where the Green Schists are developed. Nevertheless, there is good 

 reason to believe that these rocks represent different grades of 

 dynamic metamorphism of one and the same rock-type. 

 ' Q. J. G. S. No. 314. o 



