METAMORPHIC ROCKS OF SOUTH DEVON". 



725 



folia are clear and well denned, although small and at places irregu- 

 larly intercrystallized with quartz. The quartz shows minute 

 agglutinated grains, and has the appearance of having suffered from 

 pressure. In all these slides, I was on the look out for evidence of 

 a secondary cleavage-foliation, and I could trace in all the begin- 

 nings of such a structure. The thin folia have given way along 

 some of the sharp abrupt folds, as is shown in the figure given in 

 the article before quoted *, and the secondary planes thus here and 

 there arising consist of the black mineral with some mica-folia. 



B. Macroscopic Structures. 



1. Mica- and Micaceo-chloritic Schists. — In hand-specimens the 

 mica-schist gives interesting study of various forms of crumpled and 

 contorted beds. In many places examples may be found of a slipping 

 of the zigzagged layers, being an illustration of what might be 

 classed under " strain-slip " {Ausweichungs-) cleavage. In the 

 schist of the Bolt Head, Professor Bonney has described how the 

 beds are folded and contorted, and how the rock readily breaks up 

 " into rude prisms." This tendency seems due to the schist being 

 traversed by two sets of divisional planes, one parallel with the 

 original stratification-foliation, the other marking a subsequent 

 cleavage-foliation. The cleavage-surfaces have, on the whole, a 

 smoother, more continuous polish ; while the original folia are 

 thinner, closer, but more crumpled, and therefore give rise to 

 surfaces more liable to break with small irregularities. 



In certain areas the schists, whether mica- or micaceo-chloritic 

 schist, have a tendency to split along broadly undulating planes, which 

 do not entirely follow the lamination. This is a marked character 

 in much of the rock near Start Headland and at several localities in 

 the northern part of the district. Near Gullet, on the arm of the 

 estuary from South Pool, just before mica-schist sets in, the chlorite 

 rock contains what I should judge to be micaceo-chloritic bands, 

 and these are traversed by undulating planes. Thus these bands 

 have a tendency to split along curving surfaces, dividing the rock 

 into somewhat rounded rhomboids, within which the quartz often 

 thickens at places into little knots or eyes. Here the planes of 

 weakness seem to include in their course part of the slip-planes or 

 planes of cleavage-foliation, and part of what we may consider true 

 bedding-planes. There must in that case have been some modifica- 

 tion of the structure, induced by the pressure which the rock has 

 undergone ; and I thought that possibly the cause could be connected 

 with the more marked presence of quartzose layers, which might 

 have helped the bending-over of the planes of weakness, by offering 

 a resistance to the cleavage-slipping. Passing now to other ex- 

 amples of like structure, I would note a rock of mica-schist on the 

 beach near Lannacombe, whose surface, polished by the waves, 

 exhibits, with greater clearness and on a more minute scale, similar 

 markings to those of the Gullet specimen. Here, blacker patches 

 in the silvery mica-schist seem, in consequence of the structure 



* Quart. Journ. Geol. Soc. vol. 3d, 1884, fig. 7, p. 15. 



