A. R. Hunt — Devonian Rocks of S. Devon. 245 



Quartz-grains. 



The quartz-grains in the sandstones north of Beesands do not 

 call for particular notice. Just south of Beesands we have seen 

 quartz filling the interstices of a broken tourmaline, and in another 

 case a grain of tourmaline slightly affected. Professor Bonney's 

 description of a rock from the same locality seems exactly to cover 

 the case, viz. : " The rock has evidently been much compressed, and 

 some slight amount of mineral change has taken place" (Q.J.G.S. 

 vol. xl. p. 18). An occasional quartz-grain contains liquid inclusions 

 and bubbles, but without any distinguishing features. 



A slide of a Devonian grit-band from Southpool Creek (No. 22) 

 contains more than one quartz-grain of interest. One grain showing 

 considerable solution at the edges abounds in hair-like inclusions 

 together with an occasional fluid inclusion with bubble. Another 

 grain is crowded with fluid inclusions, some of which contain very 

 active bubbles, while others take the form of negative crystals. 



In a slide (No. 8) cut from the specimen of quartz-schist from 

 Start Farm, given me by Mr. A. Somervail, we find a quartz-grain 

 with hair-like inclusions ; and in another slide from the same stone 

 (No. 10) we have a grain with bubbles in negative crystals. These 

 two grains resemble the two in the Southpool Creek slide already 

 referred to. 



However, the most interesting feature in the quartz-schist lies in 

 the fact that the rock has not been submitted to pressure and heat 

 suflicient to obliterate entirely the original tourmalines and quartz- 

 grains, and that a comparison with the sandstones and grit-bands 

 of Beesands and Southpool Creek is still possible. In the case 

 of the quartz-schists at the Bolt Head, further west, tourmaline 

 is absent, and the obliteration of original structures in the quartz is 

 complete ; thus a comparison with the Devonian sandstones on this 

 particular point is not possible. 



Mica. 



On first collecting the Devonian micaceous sandstones for com- 

 parison with the Start schists, the possibility that the mica in either 

 case might be detrital had not been entertained. On Mr. Harker 

 pronouncing the Slapton Sands mica detrital,^ and that in the Start 

 quartz-schists possibly so,'^ the origin of the Devonian and metamor- 

 phic micas became a problem of importance. 



If the metamorphic rocks were micaceous before their altei'ation, 

 the presence of much of their n)ica might be accounted for with 

 much less dynamical and chemical metamorphosis than is at present 

 demanded for them. 



In the case of the sandstones and quartz-schists, it is easy to find 

 Devonian sandstones much more highly charged with mica, than 

 the Start quartz-schist under consideration, in which, as Mr. Harker 

 remarks, the mineral is only sparingly present, e.g. the sandstones 

 and grit bands of Kilmorie, Meadfoot, Slapton Sands and Beesands, 



1 Appendix, Slide, No. 3. ^ Appendix, Slide No. 8. 



