A. R. Sunt — Devonian BocJcs of 8outh Devon. 347 



in the direction of the schistose structure. Most of the chloritic mineral in the 

 rock polarizes in fairly high tints, but a portion of it gives only low interference- 

 colours. When this mineral occurs in association -with the pale hornblende, it wraps 

 round the latter in a streaky manner. A little quartz-vein which traversed the rock, 

 has been contorted and broken in the process of crushing, and affords some measure 

 of the amount of compression in the direction perpendicular to the schistosity. 



(40) A4. Coast south of site of Start Signal House. 



This is a thoroughly metamorphosed rock. In its original state it has presumably 

 been a diabase or some allied augitic type; but it is now converted into a mass of 

 minerals newly crystallized in situ, and having the fresh appearance so characteristic 

 of the crystalline schists. Original structures are completely obliterated. 



There is abundance of the pale-greenish aniphibole noticed above, partly in fibrous 

 streaks and patches, partly in compact crystals, though usually with little indication 

 of crystal-outlines. In a few places we notice crystals, bounded by the prism-faces 

 alone, with no pinacoid, and with irregular terminations ; but this is only when the 

 amphibole is moulded by a certain pale scaly mineral, apparently one of the ripidolite 

 group. The fibrous amphibole builds densely matted patches or occurs in isolated 

 wisps enclosed in the felspar areas, and having a general parallelism with the rude 

 schistosity of the rock as a whole. 



Felspar occurs abundantly in aggregates of interlocking pellucid crystal-grains, 

 rarely showing much indication of external crystal form. Twin-lamellation is very 

 prevalent, and sections perpendicular to the lamellae give extinction angles up to 

 about 14° : this is not sufficient to distinguish between varieties of albite and 

 andesine. 



After the felspar and amphibole the most abundant constituents are epidote and 

 zoisite. These form rather short columnar crystals, conspicuous, by their high 

 refractive index, with indications of a longitudinal cleavage and a marked cross- 

 jointing. The crystals give sensibly straight extinction. The two minerals are 

 distinguished by the brilliant polarization of the epidote as contrasted with the low 

 tints given by the zoisite. They occur in association with one another, and are 

 moulded or enclosed by the other constituents of the rock. So far as our knowledge 

 goes, epidote and zoisite are minerals characteristic of dynamic rather than thermal 

 metamorphism in basic igneous rocks. The amphibole and felspar might well be 

 formed in either case, but the rather coarse-grained aggregates of twinned felspars 

 in this rock would, I believe, be difficult to match among products of " contact "- 

 metamorphism. 



(3) Bi. Cliffs at North-east end of Slapton Sands. 



This is a reddish-brown, fine-grained, micaceous sandstone. As usual, the slice 

 shows the little scales of colourless mica to be much less abundant than might be 

 conjectured from the hand-specimen. They show an arrangement parallel to the 

 bedding and are doubtless of detrital origin. The grains building the bulk of the 

 rock are almost exclusively of quartz, in which both glass- and fluid-pores are 

 detected. There is a ferruginous matrix separating the grains, and in places in 

 sufficient quantity to show a well-marked lamination. An indication of "spectral 

 polarization ' ' in the quartz perhaps points to a certain degree of strain. 



(4) Cp From the same Locality, 



This rock is very similar to the preceding, though with rather less of the ferruginous 

 paste. An occasional grain of fresh lamellated felspar occurs among the quartz. 

 The little mica-flakes are rare, but there are occasional rolled granules of tourmaline, 

 both brown and blue. The quartz -grains enclose numerous flujd-pores, besides 

 brownish inclusions apparently of glass, and an occasional little prism of zircon, etc. 

 The rock has all the appearance of being derived from the destruction of a tourmaline- 

 bearing granite. 



(24) B2. North of site of Start Signal House.^ 



This is a typical dynamo-metamorphic rock. The hand-specimen shows a schistose 

 structure, accentuated by the development of a filmy secondary mica, and there are 

 1 Plate 8, fig. 1. 



