A. JR. Hunt — Devonian Rocks of South Devon. 293 



The two remaining diabases are schistose ; one, as described by 

 Mr. Harker, contains a chlorite which polarizes in higher tints 

 than usual, and is associated with hornblende.^ The other, frona 

 between Modbury and Aveton Giflfard, contains an apple-greeu 

 chlorite, lying between the laminEe of the rock, strongly pleochroic, 

 which sometimes polarizes the typical blue ; sometimes yellow, and 

 sometimes blue with yellow streaks. I have not myself come across 

 any glassy rock or any mineral corresponding with the descriptions 

 of palagonite. 



The greater part of the chlorite in the Green Rocks is the strongly 

 dichroic variety, and both in the green rocks and diabases is 

 often associated with hornblende. Ordinary chlorite occurs in the 

 quartz-schists at the Start and Bolt, in some of the felspathic veins, 

 and occasionally in the green schists with the more decidedly 

 dichroic variety. In the diabases, and in the metamorphic rocks 

 and veins, the pale chlorite seems connected with decomposition 

 by chemical action, while the deeper coloured, dichroic and more 

 brightly polarizing variety seems to be closely associated with 

 hornblende, and dynamic alteration. 



I must confess to finding the chlorites very difficult. Much of 

 this mineral in one of the diabases, when examined first by lamp 

 light, appeared so completely isotropic that I mistook it for a glass. 

 Even by daylight this chlorite often seems more colourless than 

 usual, more free from any trace of dichroism, and polarizes with 

 tints of the faintest. On the other hand the chlorite of the Schistose 

 Diabases and of the Green Rocks often tends to the other extreme, 

 being deeply coloured, highly dichroic, and polarizes in comparatively 

 bright colours. On referring to those authors who have mentioned 

 the chlorites of South Devon, their descriptions will be seen to bo 

 often qualified or guarded, e.g. Prof. Bonney — " rather a chlorite 

 than a mica," Q.J.G.S. vol. xl. p. 14, and, " a species of the chlorite 

 group," p. 16. Miss Raisin — " generally dichroic changing from a 

 feeble brownish tint to a deep green colour, Q.J.G.S. vol. xliii. p. 719. 

 and "possibly in part at least prochlorite of Dana, p. 720. Mr. 

 Harker — " apparently one of the ripidolite group," Appendix, slide 

 No. 40, and in a letter referring to one of the Devonian Quartz- 

 felspar-chlorite veins, "no doubt one of the chlorite minerals, but 

 I could not with confidence say more." Mr. Tawney, on a felspar 

 vein in the Eddystone gneiss — " vermiform collections of clinochlore " 

 Trans. Devon Assoc, vol. xiii. p. 172. 



In describing the schist from the Start Point, Miss Raisin records 

 the following facts, the correct interpretation of which is of crucial 

 importance : — " In all these slides I was on the look out for evidence 

 of secondary cleavage-foliation, and I could trace in all the beginnings 

 of such a structure." In my own investigation of the grits and 

 schists, specimens as free as possible from crumpling have invariably 

 been sought for comparison ; and as in these rocks the absence of 

 crumpling proves the absence of lateral pressure, stratification- 

 foliation and cleavage-foliation, if present, are likely to be coincident 

 ^ Appendix, slide 33. 



