HISTORY OF GRITS AND SANDSTONES. 



15 



mainly composed of well-rounded siliceous grains, may be cited a 

 grey, friable, fine-grained rock from the base of this formation, 

 which is exposed in the railway- cutting at the Runcorn station, 

 and a dark-red sandstone of still finer texture, belonging to the 

 Irodsham beds, at no great distance from the same locality. The 

 Lower Keuper cupreous sandstones of Alderley Edge, Cheshire, are 

 frequently made up of quartz crystals on which still more minute 

 crystals of vanadinite may sometimes be distinguished by the aid 

 of a lens. 



In many localities the quartz pebbles occurring in crystalline 

 arenaceous rocks have their surfaces, and more particularly their 

 upper surfaces, covered by minute crystals of that mineral. This 

 may be observed in the case of pebbles found in Lower Keuper 

 sandstone in a quarry near Litherland. 



At Dymoke, Worcestershire, there is a Lower Keuper sandstone 

 which is sufficiently coherent to admit of the preparation of thin 

 sections. This is a fine-grained, quartzose, distinctly laminated 

 rock, of which the component grains are usually about ^4-^- inch 

 in diameter. These to a large extent consist of quartz, sometimes 

 enclosing hair-like crystals of rutile, and occasionally fluid- cavities, 

 in some of which bubbles were observed. A certain amount of 

 felspar, a portion of which is triclinic, is present in this rock. A 

 few flakes of colourless mica, and a little of the- fibrous mineral 

 which has been referred to as often occurring in the cement of 

 certain sandstones, were also observed. The cementing material, 

 which contains a little kaolin, encloses a few minute garnets, and 

 is frequently stained by hydrated ferric oxide. 



The Waterstone beds belonging to this series enclose numerous 

 angular fragments of dark-coloured slaty rock, some of which are as 

 much as \ inch in diameter. The quartz grains, many of which are 

 inch in diameter, are usually much rounded, and not unfre- 

 quently enclose fluid-cavities. In addition to quartz and the 

 cementing material felspar is present, as is also, in small quantities, 

 another mineral of a light yellowish-green colour, which I have 

 been unable to identify, but which occasionally forms part of the 

 cement. 



A fine-grained sandstone of Upper Keuper age, which occurs at 

 High House, Warwickshire, is to a large extent composed of quartz 

 crystals, while a bed of loose sand, found 25 feet below the surface 

 at Frodsham, above the Keuper Marl, is, on the contrary, entirely 

 made up of much-rounded grains. 



The rounded quartz grains of Triassic sandstones, when examined 

 in a suitable medium, after the removal of their external ferruginous 

 coating, are found to be colourless and often transparent. Grains con- 

 taining fluid-cavities are comparatively rare, but they are apparently 

 more numerous in the Keuper sandstones than in rocks of Bunter 

 age. A few crystals of schorl occasionally present themselves in the 

 quartz of these rocks, which is not unirequently rendered turbid 

 by the enclosure of what is probably a little ferruginous clay. It 

 is, however, probable that the grains containing fluid-cavities in 



