208 THE SEDIMENTARY ROCKS 



withstand high temperatures and cannot be used for the lining of 

 furnaces, because the iron, alkalies, and alkaline earths which they 

 contain cause the bricks to disintegrate. 



Fire-day is a nearly pure mixture of sand and clay, with only 

 traces of iron, magnesia, or lime, and therefore burns to white 

 or buff-coloured bricks, which will resist very high temperatures. 

 Fire-clays occur frequently beneath coal seams, representing the 

 ancient soil in which the coal plants grew. Such ancient fire-clays 

 are often hard rocks, and must be ground up before using. 



Mudstone is a rock which is composed of solidified clay or fel- 

 spathic mud, or a mixture of the two, and which crumbles rapidly 

 into mud when exposed to the action of the weather. 



Shale is a finely stratified or laminated clay rock, formed from 

 the solidification of mud and silt. In some of the paper shales 

 there are as many as thirty or forty laminae to the inch, each rep- 

 resenting a separate process of deposition. Shales ordinarily con- 

 tain more or less sand, and as this increases in quantity, they shade 

 gradually into arenaceous shales and argillaceous sandstones, or 

 by the increase of calcareous matter into limestones. Bituminous 

 shale is coloured very dark or black by the carbonaceous matter 

 with which it is saturated. When distilled, the bituminous shales 

 yield hydrocarbons, and are of considerable economic importance ; 

 the carbonaceous matter may be of either animal or vegetable 

 origin. Shales of this class grade into coals. 



Marl is clay containing carbonate of lime, which rapidly crum- 

 bles on exposure to the weather. 



2. CHEMICAL PRECIPITATES 



Rocks which have been principally or entirely formed by chemi- 

 cal processes are, for the most part, of locally restricted extent, and 

 are not at all comparable to the great masses of mechanical and 

 organic sediments. This arises from the fact that the chemical 

 processes occur in a conspicuous way only around the mouths of 

 certain classes of springs (p. 128), and in closed bodies of water 

 without outlet and subject to evaporation (p. 149). 



The chemical precipitates may be classed under the following 



