40 DR. STAMP A>~D ME. WOOLDBIDGE OX THE [vol. lxxix, 



rounded grains are frequently to be found. Much of the quartz 

 is traversed by curved fractures. In some cases a crystal has been 

 broken in place along such fractures, and the fragments separated 

 by matrix. Since the rock is extensively silicified, it may be 

 possible to refer this shattering to strains set up during the crystal- 

 lization of the secondary quartz, or alternatively to the setting of 

 a siliceous gel. The quartz-grains show beautiful secondary out- 

 growths as a result of the silicification. 



Before leaving the subject of the larger quartz-grains, it may be 

 well to refer at greater length to the curved fractures mentioned 

 above, which give rise to concave outlines. Despite a somewhat 

 loose current usage, we cannot apply the terms bogen-struktur 

 or aschen-struktur in this case. These terms, introduced by 

 O. Miigge, 1 seem to have been used explicitly by him with refer- 

 ence to the concave fragments produced by the breaking-up of 

 glassy vesicular rocks. Moreover, Pirsson, by proposing to sub- 

 stitute the term vitroclastic, which is self-explanatory, clearly 

 recognizes the difference between the concave outline of a broken 

 vesicle and conchoidal fracture. 2 At the same time, it does appear 

 that these curved fractures are especially common in tuffs. Many 

 cases of perfectly straight fractures have been observed in true 

 rbyolites, although the exact mechanical differences which deter- 

 mine the formation of a straight or a curved fracture are difficult 

 to define. It is necessary to recall in this connexion that Prof. 

 W. W. Watts 3 proved, many years ago, that true perlitic cracks 

 could be formed in the quartz-phenocrysts of a porphyritic pitch- 

 stone. 



A further interesting feature of the quartz-fragments is the 

 frequent occurrence of the ' resorption -inlets ' so commonly seen 

 in rhyolites (PI. I, fig. 5). Prof. A. H. Cox has described a 

 similar feature in the tuffs of Abereiddy Bay, 4 and Dr. J. S. Flett 

 describes examples from the quartz-keratophyre tuffs of Devon. 5 



The felspar-crystals attain a greater size than do those of quartz. 

 They are almost invariably altered to an opaque substance, probably 

 of the nature of kaolin, which outlines the cleavages and covers 

 the surfaces generally. A few flakes of secondary mica are 

 occasionally developed. 



While sometimes showing good form, the felspar frequently 

 presents the appearance of having been breceiated in place like the 

 quartz. Angular pieces are seen to surround some larger fragment, 

 and their outlines show clearly that they once formed part of it. 



Most of the felspar is undoubtedly albite, and some of it seems 



1 Neues Jahrb. Beilage-Band viii (1893) p. 648. 



2 Amer. Journ. Sci. ser. 4, vol. xl (1915) p. 198. 



3 Q.J. G. S. vol. 1 (1894) p. 367. 



4 Ibid. vol. Ixxi (1915-16) pp. 298 et seqq. 



5 ' The Geology of the Country around Newton Abbot ' Mem. Geol. Surv. 

 1913, p. 58. 



