A. R. Hunt — Devonian Bocks of South Devon. 291 



rounded the matrix contains much fibrous hornblende. The 

 belonites in felspar are such a prominent feature in these rocks 

 that I have carefully searched the more chloritic of the Devonian 

 diabases for any indication of the incipient formation of such 

 microliths in felspar, but hitherto without success. In the more 

 altered Winslade rock, B, there are several clear grains with 

 belonites; but, on comparing this slide with the one from Eickham 

 Sands, Mr. Teall considered the grains in the diabase to be quartz, 

 but was unable to detect quartz in the metamorphic rock ; twelve 

 grains taken at random proving to be in each case felspar. 



Professor Bonney has, however, described quartz with belonites 

 from the Prawle district, associated with a mineral supposed to be 

 kyanite^ (Q.J.G.S., vol. xl. p. 17), Miss Eaisin has also described 

 " a typical slide of chlorite schist," as containing grains with 

 belonites, the grains being attributed to quartz (Q.J.Gr.S., vol. xliii, 

 p. 719). 



My evidence on this point is merely negative. Quartz grains 

 with belonites appear to be absent from my slides of the green 

 rocks, the water-clear grains being felspar. If such quartz-grains 

 do occur, and we have the evidence of Prof. Bonney and Miss Eaisin 

 that they do, it is possible that the early stage of their formation 

 may be revealed in the Winslade Devonian diabase referred to above. 



Felspak and Felspathic Veins. 



In 1839 Sir Henry de la Beche reported the occurrence of gneiss 

 near the Prawle Point, by the addition of felspar to the mica 

 and quartz of the ordinary schist.^ This statement has been the 

 source of much perplexity to geologists. In 1881 the late Mr. B. B. 

 Tawney declared, " there was nothing approaching gneiss there." ^ 

 In 1883 Prof. Bonney suggested that a mica-schist banded with thin 

 quartzose laminae might be De la Beche's gneiss.* In 1891 Mr. W. 

 A. E. Ussher called attention to a grey rock near the Start with 

 incipient foliation, presenting a slightly gneissoid appearance.^ 

 Messrs. Sedgwick, Murchison and Pengelly have each in turn ex- 

 pressed their doubts as to the occurrence of gneiss near the Prawle.** 



Sir Henry de la Beche defines precisely the rock he considers 

 gneiss, viz. a rock composed of mica quartz and felspar. Thus 

 Prof. Bonney's quartz-schist will not meet the case for lack of 

 felspar, just as the grey gneissoid rock near the Start'' must fail for 

 lack of quartz. 



The importance of De la Beche's observation consists in the 

 recognition of felspar in the mica- and quartz-schists. This felspar 

 occurs either as laminae in the schists, or as veins, the latter being 

 sometimes associated with quartz and chlorite. 



' I regret to say that my own identification of kyanite in the Bolt Schist (Trans. 

 Dev. Assoc, vol. xxi. p. 260) was wrong. I am authoritatively informed that there 

 is no kyanite in any of my slides from that district ; the mineral I mistook being 

 probably felspar. 



2 Eeport Cornwall and Devon, p. 27. ^ Brit. Assoc. Rep. 1891. 



3 Trans. Dev. Assoc, vol. xxi. p. 469. '^ Trans. Dev. Assoc, vol. xi. p. 322. 

 * Q.J.G.S., vol. xl. p. 7. '' Appendix, slide 40. 



