12 



J. A. PHILLIPS ON THE CONSTITUTION AND 



amount of abrasion subsequently to their formation, not a point being 

 broken or an angle removed. Sometimes the original grains of 

 quartz have, by the subsequent deposit of silica upon their surfaces, 

 become converted into complete double-terminated crystals ; but 

 the forms are frequently less simple, and the faces bounding the ex- 

 terior cannot all be referred to the same crystal. That this crystal- 

 lization is produced by a deposit of silica around the original grains 

 of quartz, subsequently to their having become members of an accu- 

 mulation of sand, was first noticed in British rocks by Professor 

 Bonney*, and has been subsequently demonstrated by Dr. Sorbyf. 

 It is almost equally certain, as will be subsequently shown, that a 

 portion, at least, of the silica so deposited has been derived from the 

 decomposition of felspar. 



Among highly crystalline Carboniferous sandstones may be men- 

 tioned one belonging to the Mountain-Limestone series, which occurs 

 at Yeathouse, in Cumberland, that of Parkhead, in the same county, 

 and that of Augill, near Brough, Westmoreland; the two last of 

 Yoredale age. Another sandstone in the Lower Coal-measures, 

 worked at Barngill quarry, in the county of Cumberland, is also 

 crystalline. 



Permian. — The St. Pees Sandstone, at Eheda, Cumberland, is a 

 fine-grained reddish-brown rock, composed of a mixture of angular 

 fragments and minute crystals of quartz with a little felspar, the 

 whole being united by a cement rendered, to some extent, opaque 

 by ferric hydrate. The grains and crystals of quartz have usually a 

 diameter of about -gi - i n °h, and contain but few fluid- cavities. The 

 colour of this sandstone, like that of the majority of similar rocks, is 

 caused by a coating of hydrated oxide of iron over the surfaces of 

 the grains and crystals of which it is composed, but which is readily 

 removed by digestion in acids. Felspar is present in notable quan- 

 tity, and is often considerably altered ; no triclinic species was 

 observed. A small amount of colourless mica is present in the form 

 of water-worn flakes, together with a few imperfect crystals of 

 schorl. 



Numerous other crystalline sandstones of Permian age might be 

 cited ; but those of Penrith, which have been described by Dr. Sorby, 

 are probably the most interesting and remarkable examples. 



Triassic. — Among the Bunter sandstones of Lancashire and Che- 

 shire are certain reddish-brown friable beds, possessing but little 

 cohesion, and of which the constituent grains are all so completely 

 rounded, that the disintegrated sand flows between the fingers as 

 readily as shot. Deposits of such sandstones, which are distin- 

 guished by the name of " Millet-seed beds," occur plentifully in 

 the Lower Mottled series, and occasionally among the Upper Mottled 

 sandstones, as well as in the Frodsham beds of the Keuper. 



A specimen of millet-seed sandstone from the Lower Bunter, 

 obtained in the form of a core, at a depth of 1039 feet, from the 



* Quart. Journ. Geol. Soc. vol. xxxv. p. 666. 



t Address delivered at Anniversary Meeting of the Geological Society of 

 London, 20th February, 1880, p. 36. 



