96 PRIMARY AND SECONDARY SANDSTONES. 



In the Carboniferous system, Mr. Phillips has examined a fine- 

 grained sandstone in Cumberland belonging to the Yoredale series. 

 The quartz is angular and generally free from fluid cavities. When 

 cavities do occur they are full ; the rock also contains a little felspar 

 and kaolin, and some white mica. The Millstone Grit in Cumberland 

 consists almost entirely of grains of quartz T i-th of an inch in 

 diameter, bound together with a silicious cement, and containing a 

 little felspar. The quartz includes occasional fluid cavities and a few 

 needles of tourmaline. The sandstones of the Lower Coal Measures 

 near Bradford consist in the main of fragments of quartz and felspar ; 

 the quartz sometimes encloses tourmaline, but contains few fluid 

 cavities ; the felspar is chiefly triclinic, and is associated with some 

 kaolin, a few garnets, and flakes of dark and colourless mica. The 

 fragments in this rock are -5-50^ of an inch in diameter; but many of 

 the Carboniferous sandstones are chiefly formed of quartz grains which 

 have crystallised in the positions in which they now occur, for they 

 show no sign of abrasion, not having lost a point or an angle. 



The Permian sandstone of Cumberland, which has a reddish tinge 

 owing to the opaque ferric hydrate in the cement, is a mixture of 

 angular fragments and minute crystals of quartz and a little felspar ; 

 the quartz grains contain few fluid cavities, and have a diameter of 

 g-J^th of an inch. The flakes of colourless mica are water-worn. 



The Triassic sands vary considerably. Some of the Eunter sand- 

 stones of Lancashire and Cheshire, known as " Millet-seed Beds," 

 flow between the fingers like shot ; these have most of the grains 

 rounded like miniature pebbles ; their diameter is from -^jth to TrJ^th 

 of an inch ; they are partly quartz, partly felspar. The quartz grains 

 are frequently covered with transparent crystalline silica. Such 

 beds may well be blown sands, like those of existing deserts, 

 united by a ferruginous cement which has invested the grains. 

 In many of the beds the quartz is almost entirely in the form of 

 minute crystals; such a rock is well seen near Ormskirk. The 

 Upper Trias or Keuper generally consists of well-rounded silicious 

 grains. At Dymoke, in Worcestershire, the quartz sometimes encloses 

 crystals of rutile and fluid cavities with bubbles ; part of the 

 felspar in this rock is triclinic, and there are a few flakes of white 

 mica. In the cement are minute garnets and a little kaolin. The 

 beds called water-stones include angular fragments of dark-coloured 

 slate sometimes half an inch in diameter ; the quartz grains are often 

 y^th of an inch in diameter, much rounded, and contain fluid cavities ; 

 there is some felspar. Mr. Phillips believes that the grains in 

 Triassic sandstones which contain fluid cavities and crystals of 

 tourmaline are from a different parent rock to those grains from which 

 the cavities are absent. 



The Upper Lias sand in Gloucestershire has a calcareous cement, 

 the quartz grains in which are generally angular, though some of the 

 angles are a little rounded ; they are -s-^th of an inch in diameter, 

 sometimes enclose fluid cavities without bubbles, and are associated 

 with numerous fragments of tourmaline and garnet, and probably a 



