726 



MISS C. A. RAISIN ON THE 



described, to tail off where the cross planes break the lamination. 

 Very similar to this is a micaceo-chloritic rock from the shore 

 opposite Gullet, which I examined with the microscope. In 

 the hand-specimen blacker bands continually tailing off seem to 

 cause an incipient formation of " eyes " of the darker material. 

 These bands, under the microscope, are found to consist of chlorite, 

 mica, and a quantity of a black mineral present in all my slides 

 of micaceo-chloritic rock, which both here and in some others consists 

 markedly of small crystalline masses rather flattened in form, and 

 ranging along the lamination, like the titaniferous iron-ore shown 

 in the Scourie-Dyke schist*, only the ferrite in my slide i^ more 

 abundant. Here (as well as in one other slide) I thought it 

 accompanied by grains of sphene. Some of the micaceous layers 

 present an appearance as if they were flowing around the larger 

 grains of quartz ; and the hand-specimen, like others of the micaceo- 

 chloritic group, has a very squeezed look. The slide has the structure 

 already noted as generally belonging to the group — the small mosaic 

 of quartz granules surrounding larger grains which are sometimes 

 broken across ; and the aggregations of microliths are also present. 

 This rock was obtained near Westercomb, from within 60 yards of 

 a junction with the phyllites. In other northern localities of the 

 district the curving fracture-planes, as seen in hand-specimens, are 

 very marked in schists which have what seemed to me a specially 

 slickensided and crushed look ; and this occurs where I am inclined, 

 from stratigraphical relations, to suspect that a line of fault occurs, 

 which has very possibly split and followed two directions, as, for ex- 

 ample, in the point opposite Westercomb. 



On the whole, the characteristic of the mica-schist of South Devon 

 seems to be a tendency to develop cleavage-planes of less or greater 

 force, which become, at places, fairly well marked, although not in 

 an equal degree with the original foliation-planes. 



2. Chlorite-Schists. — Certain structures in the chlorite- schist, when 

 it is studied in the field, seem to require some notice ; but I am very 

 diffident about making any suggestions as to their relations. Apart 

 from any ferruginous decomposition, the chlorite in weathering 

 acquires often a paler shade, sometimes almost whitish, and some- 

 times a delicate pale sea-green. This last colour I generally found 

 occurred in parts protected from rain-wash, and I have never seen a 

 more beautiful study in rocks than that in some of these chlorite-masses 

 where, receding slightly beneath a projecting ledge, they expose a 

 surface coated with the soft pale sea-green dust. The picturesque 

 appearance is increased by the tendency to pitted weathering here, as 

 elsewhere, exhibited, which has already been described by Professor 

 Bonney, In other exposures, I noted a tendency to rather regular 

 rhomboidal divisions. These two structures I should suppose nearly 

 related ; but the rhomboids at all events seem to be due to the develop- 

 ment of two obliquely crossing sets of planes, the one set being in the 

 direction of bedding, the cause of the other set being more difficult 

 to prove. It might perhaps be a kind of jointing ; but there is some 



* Teall, ' British Petrography,' pi. xx. fig. 2. 



