HISTORY OF GRITS AND SANDSTONES. 



11 



description of it is unnecessary. The felspar, of which a large pro- 

 portion is triclinic, is present in large quantities ; and, as in the 

 case of the other specimens examined, the quartz contains but few 

 fluid-cavities. It will be observed that in this respect the quartz of 

 these grits materially differs from that of the Cornish granites, in 

 which fluid-cavities with bubbles are abundant. 



Carboniferous. — A fine-grained yellowish- white sandstone from 

 Shalk Beck, Cumberland, belonging to the Yoredale series, much 

 spotted by stains of hydrated ferric oxide, and rendered somewhat 

 mealy by the presence of kaolin, contains but few fragments of more 

 than yi^ inch in diameter. The quartz is in angular pieces, colour- 

 less and transparent, and, to a large extent, free from fluid-cavities, 

 which, when present, are, for the most part, full, and consequently 

 without bubbles. In addition to quartz, with a little felspar and 

 kaolin, the only recognizable mineral is white mica. For the 

 analysis, see p. 21. 



A sandstone from Brigham, Cumberland, belonging to the Mill- 

 stone-Grit series, was examined both microscopically and chemically, 

 and is essentially composed of fragments of quartz with a little fel- 

 spar, the grains being usually about y-1^ inch in diameter, united by 

 a siliceous cement, which is sometimes a little cloudy. Between the 

 constituent fragments of this rock there are sometimes minute 

 crystals of a mineral which may perhaps be epidote ; and the quartz, 

 which is colourless and transparent, encloses a few needles of tour- 

 maline, besides containing occasional fluid-cavities, but few of which 

 contain bubbles. For the chemical composition of this sandstone, 

 see page 21. 



At Spinkwell quarry, near Bradford, a foliated siliceous sandstone, 

 which can be raised in the form of very large slabs, is worked in the 

 Lower Coal-measures, and is much used in the construction of chemical 

 apparatus on a manufacturing scale. This sandstone is mainly com- 

 posed of fragmentary quartz and felspar, of the latter of which a 

 portion is triclinic, united by the usual siliceous cement. The 

 quartz is colourless and transparent, and contains but few fluid- 

 cavities, although it sometimes encloses needles of schorl. 



In addition to quartz and felspar, this rock contains kaolin, with a 

 few minute garnets, and flakes of dark-brown and colourless mica, 

 which are more abundant along certain lines of foliation than else- 

 where. A few minute crystalline scales of micaceous oxide of iron 

 were observed in the quartz of this sandstone. The component 

 fragments rarely exceed -rri- - inch in diameter. For analysis of this 

 rock, see page 21. 



The tine-grained yellowish-grey Coal-measure sandstone of Stony- 

 hough, Workington, Cumberland, is, to a large extent, composed of 

 minute crystals or crystalline aggregations of quartz of about -r^- 

 inch in diameter, somewhat loosely united by a siliceous cement, 

 often much stained by hydrated ferric oxide. 



A considerable number of sandstones belonging to the Carboni- 

 ferous period are chiefly composed of quartz crystals, which have 

 evidently crystallized in situ, since they exhibit the freshness of out- 

 line peculiar to crystals which have not been subjected to the slightest 



