294 A. M. Hunt — Devonian Rods of South Devon. 



In the slide described by Mr. Harker' it will be observed that 

 the little cracks cemented by opa(|ue iron-ores maintain a rough 

 jiarallelism, agreeing in direction with the scales of mica. The hand- 

 specimen shows also the general parallelism of the quartz-lamin£e. 

 This rock is clearly a modified micaceous grit-band in which all 

 the minerals, iron, quartz, and mica, tend to lie in planes parallel 

 with the original bedding of the band. 



Now if we turn to an equally uncrumpled Devonian micaceous 

 grit, we find a rock macroscopically extremely' like the quartz -schist, 

 with well-defined lines of sedimentation in which the iron -ores and 

 minute flakes of mica as seen in the microscope take a decided linear 

 arrangement ; the whole rock being also micaceous. Sometimes these 

 Devonian grits, through pressure, crack and gape in the planes of 

 stratification, sometimes obliquely across these planes ; such cracks 

 being occasionally filled by quartz, without the grit itself being 

 afi'ected by solution. Now if these grit bands with their already 

 existing iron- and mica-lines were exposed to sufficient heat and 

 pressure for their constituent quartz-grains to be dissolved, the 

 mica-flakes distributed through the rock would necessarily float 

 out into the quartz, and the two together would fill and cement 

 cracks dependent in direction on the direction of pressure ; so that 

 these new cracks would (in the majority of cases) not be parallel 

 witli the original stratification-foliation of the grit. This simple 

 process seems to be indicated in the less altered quartz-schists of 

 the Start, in which to judge by analogy with the Devonian grits 

 the black lines of iron and mica usually correspond with the original 

 sedimentary lines, while the quartzose laminae are in nearly all cases 

 of subsequent date. 



It may be noted that among the minerals accompanying and 

 determining foliation in the mica- and quartz-schists of South 

 Devon are iron, mica, quartz, and felspar, and in the green rocks 

 felspar, hornblende, and chlorite. In the Devonian Volcanics the 

 lamina3 of the schistose rocks are often determined by hornblende 

 and chlorite, but I have not noticed any distinct alineation of 

 felspar, as distinguished from veins. 



EXPLANATION OF PLATE VIIT. 



EiG. 1. — Red Mica Schist, north of Start Signal House. (24) Magnified 13 

 diameters. Original streaks of red and black iron ores traverse the slide 

 in places obliquely from top to bottom. These are intimately associated 

 •with mica, which lies between them in wavy lines. These more ancient 

 minerals are invaded by quartz, which traverses the slide from side to 

 side. The mica-flakes disengaged by the quartz tend to align themselves 

 flake by flake in the general direction of the quartz. In this rock, though 

 a mica schist, the planes of schistosity are indicated by iron and quartz, the 

 direction of the mica being uncertain. 



Fig. 2. — Chlorite-schist, Rickham Sands, Salcombe. Magnified 31 diameters. 

 Secondary felspar-granules with belonites in a chloritic matrix. The 

 schistose character of the rock is determined by the general alignment of 

 the felspar-granules. 



{To be continued in our next Number.) 

 ^ Appendix, slide No. 8. 



