202 E. HILL AND T. G. BONNEY ON THE 



felspar crystals, both orthoclase and plagioclase, with lapilli and 

 fragments of altered rocks, generally slates. The lapilli are un- 

 mistakably of igneous rock, irregular in form, doubtless volcanic ; 

 sometimes they resemble a glass, rendered more or less opaque by 

 impurities (chiefly ferruginous), sometimes they are crowded with 

 minute plagioclase crystals, and are doubtless andesite, sometimes 

 they exhibit (with crossed Nicols) an imperfectly crystalline mosaic- 

 like structure common in many trachytes*. Subangular, and some- 

 times rounded, fragments of quartz are present in many cases ; grains 

 of magnetite or ilmenite are frequently scattered about ; viridite and 

 epidote are often abundant as secondary products. 



We proceed to indicate briefly the peculiarities of the various 

 sections which we have examined. 



Forest-RocJc-Inn Grit (p. 773). — Much viridite in ground-mass ; 

 fragments mostly felspar, generally, as it seems, orthoclase ; these 

 contain numerous minute endomorphs, bright- coloured with crossed 

 Nicols, doubtless decomposition products ; some quartz fragments ; a 

 few doubtful, whether lapilli or altered rock. The rock is probably 

 not pyroclastic. 



JRocTc from Spinney north of Old John (p. 764). — Much frag- 

 mental felspar of both kinds, altered as above; a little quartz; 

 lapilli of both kinds ; also a few fragments of slate. A pyroclastic 

 rock. 



Bawdon- Castle Grit (p. 783). — Ground-mass largely composed of 

 minute felspar crystals, in form resembling plagioclase ; larger 

 broken crystals of both kinds ; lapilli, some the impure glass, some 

 andesite, one or two showing plagioclase crystals very clearly ; 

 crossed prisms show a bright- coloured fibrous mineral with the 

 viridite (possibly a variety of hornblende) ; a little quartz and not 

 much epidote. A pyroclastic rock. 



Grit-bed, summit of Broombriggs (p. 759). — A little finer than 

 the last, and rather more altered, and with rather more quartz 

 grains of irregular outline ; lapilli less distinct, of trachytic aspect, 

 but possibly, in some cases at least, only highly altered slate frag- 

 ments. 



There is a certain resemblance between these two rocks which 

 may be significant, though it is hardly enough to enable us to class 

 the former (of whose horizon we are ignorant) with the latter. 



Grit from Quarry near Forest Gate, Loughborough (p. 755). — 

 Ground-mass rather streaky, containing often much dark dust; 

 many fragments ; felspars of both kinds ; rather clear quartz, sub- 

 angular in form ; and numerous lapilli, mostly anclesitic ; a few 

 fragments of an indurated slaty rock. 



Agglomerate near Woodhouse Mill (p. 759). — A pyroclastic rock 

 as above. The lapilli appear to be mostly andesite ; one contains 

 six or seven very distinct plagioclase crystals, the largest about 



* It is of course one of the forms of microcrystalline structure. We shall 

 refer to fragments exhibiting it as " trachyte," because it occurs (as specimens 

 from Hungary show) in trachytes of all kinds, but shall call those consisting of 

 a glass crowded with minute plagioclase distinctively andesite. 



