METAMOKPHIC ROCKS OF SOUTH DEVON. 



723 



planes,, and that the grains themselves are sometimes outlined 

 approximately parallel to the curving lines within. I tried to 

 observe the optical characters of the belonites, but not always 

 successfully; some indications seemed to agree with characters of 

 sillimanite — the clearness of the mineral, and its absence of colour, 

 its occurrence in long slender prisms with transverse cracks, and 

 occasionally the appearance of bright colours with crossed nicols ; 

 the extinction in some examples seemed to occur at an angle of 

 about 20°, but varied in others. 



Frequently in the masses of chlorite -schist seen in the field, 

 there occur thin bands of chloritic rock of brighter green colour 

 and with, very smoothed surfaces as if slickensided : even in freshly 

 hewn quarries these bands were generally too fragile to yield slides 

 for the microscope. One firmer band, however, having a more 

 silvery micaceous look, occurs in a quarry hy the new road near 

 Snapes Point, and may perhaps be representative. This gives a 

 clear, filmy, apple-green chlorite, and very clear large mica-flakes 

 accompanying an exceptionally small quantity of quartz, most of it 

 exhibiting signs of secondary change. There are numerous grains, 

 some of epidote, some of garnet, and others of a mineral rather 

 resembling garnet, but apparently not isometric, giving a dull purple 

 tint on rotating the stage. The layers are crumpled, the mica- 

 flakes lying at intervals obliquely across the general direction, as if 

 bent down in the slipping. 



When we turn to the examination of the most general type of 

 micaceo-chloritic schists, we find some of the best specimens near 

 " Snapes" Point," Scoble "Point, and Westercomb, all places approach- 

 ing the northern boundary-line. Macroscopicalfy, these rocks differ 

 from the ordinary mica-schists in being duller in appearance, although 

 rather light in colour, greyish, or sometimes with a slightly greenish 

 tint. In the mass they are generally more evenly bedded. Under 

 the microscope these rocks are found to consist chiefly of mica, 

 chlorite, and quartz, with possibly felspar and a mineral, more or less 

 abundant, approximating in its cleavages to kyanite, several grains 

 showing interrupted cleavages meeting at an angle of about 56°, and 

 reminding one of the planes parallel to the base and the brachydome 

 (0 P and Pa) shown by Max Bauer *. Grains are fairly abundant 

 in these slides having a marked twinning, some quite simple, others 

 slightly repeated ; and in one or more examples the crystal is cracked 

 and its parts displaced. The quartz is dirty from the number of 

 minute enclosures, and has the usual schistose structure, irregularity 

 ill the size of the grains and shading of their colours in polarized 

 light suggesting subsequent strain of the rock. In many places, 

 larger clear grains are imbedded in a mosaic of small shaded ones 

 and not uncommonly the large grain is cracked across, and the 

 commencing development of the fine mosaic can be traced along the 

 crack. In some slides occur certain granular aggregations which 

 are partly resolvable under a higher power of the microscope into 



* Zeitschrift der Deutscben geologischen G-esellschaft, 1878, Bel, 30, Taf. xiv. 

 fig. 1 a. 



