part 2 J THE METAMORPHOSED BOCKS OF THE START AREA.' 185 



'Green Schists,, quartz is never a prominent constituent, but is 

 usually present in minor amount associated with albite-grains. 



The presence of abundant quartz and muscovite is characteristic 

 of the composite rocks dealt with below. 



Biotite enters as a subordinate constituent of a Green Schist 

 south of Moor Sands, west of Prawle Point. This is the more 

 noteworthy, on account of the remarkable rarity of this mineral in 

 the rocks of the immediate neighbourhood of Start Point. It is 

 developed in flakes associated with chlorite. Some muscovite is 

 similarly associated. The pleochroism of this mica is pale yellow 

 to dark brownish-green, and it is practically uniaxial. It is in 

 this rock that nodular masses of epidote, calcite, and chlorite are 

 developed, the nature of which will be considered on a later page. 



(ii) Hornblende-Epidote-Albite-Schists. 



By increase in the amount of hornblende in these rocks, and 

 'concomitantbya decrease in the quantitative proportion of chlorite, 

 the Green Schists pass into the second well-defined type of the 

 hornblende-epidote-albite-schists. There is no doubt that these 

 rocks represent an increasing grade of metamorphism of one and 

 the same rock-type. They can be recognized from all parts of the 

 area where the Green Schists are exposed. 



The petrographic character of this type of Green Schists can be 

 indicated by the description of a rock which occurs at Prawle Point 

 close to the signal-station. Macroscopically, it is a grey-green, 

 not markedly schistose rock, showing porphyroblasts of a dark- 

 green cleavable mineral projecting on weathered surfaces. Under 

 the microscope the constituents seen to be present are hornblende, 

 epidote, albite, titanite, and chlorite. The principal constituent is 

 a colourless to grej^-green hornblende, present as porphyroblasts 

 measuring up to P5 mm. in length. It is only weakly pleochroic 

 in pale green tints. Hornblende is also developed as cross-fractured 

 fibres, as in the chlorite-epidote-schists. The maximum extinction - 

 angle is 22°, and the birefringence =-022. The periphery of 

 these porphyroblasts is often enclosed by aggregates of epidote- 

 granules, with which titanite-granules are mingled. The optical 

 character of the hornblende is negative. 



Epidote occurs in subidioblastic grains of quite small dimen- 

 sions. This is a less ferriferous type than that represented in the 

 chlorite-epidote-schist previously described. The birefringence 

 = '028, and there may be zonal structure developed. In such 

 cases the shell is more ferriferous than the core. 



Albite rarely forms porphyroblasts in any of these rocks, but 

 tisually is interstitially developed. It may be a less sodic type 

 than the albite of the chlorite-epidote-schists. The refractive 

 index is only slightly lower than that of Canada balsam. Cleavage 

 is rarely developed, and the same remark applies to twinning. It 

 is clear that some of this felspar is as calcic as acid oligoclase, for 

 some of the slides show grains with the refringence of Canada 

 balsam, and give a negative optic figure. 



