46 CLAY; SELENITE, ALUM. 



and many large towns. Sands and sandstones occur in each of the 

 great divisions of the geological series in this country. Among 

 them may be named Harlech Grits, Llandovery beds, Downton 

 Sandstone, much of the Old Ked Sandstone formation, much of the 

 Millstone Grit, the Pennant, and many sandstones associated with 

 the CoaL The Permian and Triassic rocks in this country are chiefly 

 sandstones. The Lower Oolites in Yorkshire are largely formed of 

 sandstone. There are also the Portland Sands, sands of the Purbeck 

 and Wealden periods, Lower and Upper Greensand, the Thanet Sands, 

 the Bagshot Sands, and other deposits. Sand and sandstones generally 

 form somewhat elevated and dry country, which is frequently wooded, 

 especially with fir, and is sometimes covered with heather. See p. 92. 



Clay. Clay consists chemically, chiefly of silicate of alumina, and 

 has very nearly the same composition as the mineral felspar, which 

 makes up so large a part of fire-formed rocks. Sometimes when hard- 

 ened by pressure, and by containing other minerals, the clay is called 

 shale ; it then splits into thin layers in the direction in which it 

 was deposited. Clay consists of extremely fine particles which can 

 easily be transported by moving water, as may be seen by the muddy 

 state of rivers after rain in clayey districts. The colour of clay is 

 generally due to some oxide of iron ; it is usually grey or blue, some- 

 times brown, occasionally white, yellow, red, crimson, purple, violet, 

 or black. Clay generally forms valleys and low land ; it does not 

 easily allow water to pass through it, but always holds a good deal of 

 water suspended in its substance ; and when this is evaporated, large 

 and deep surface cracks and fissures are formed, which may be enlarged 

 by rain into gullies. The older British clays have undergone certain 

 changes, so that it is convenient to give them another name, and 

 they are now termed slates and slate rocks. Some of the well- 

 known clays are the Lias, Bradford Clay, Oxford Clay, Ampthill 

 Clay, Kimmeridge Clay, Wadhurst Clay, Weald Clay, Gault, London 

 Clay, Barton Clay, and Boulder Clay. These deposits are often well 

 wooded, especially with oak, beech, and elm. See p. 99. 



Selenite. Clays sometimes contain a large amount of iron pyrites, 

 which is usually a yellow brassy-looking mineral, consisting of sulphur 

 and iron. When it decomposes, the sulphur is set free in the form 

 of sulphuric acid. This acid, taken up by water, usually dissolves the 

 carbonate of lime of shells, and then the new compound crystallises 

 and forms transparent crystals of the mineral selenite, which is a 

 variety of gypsum, and is composed chemically of hydrated sulphate 

 of lime. The water may be driven off from this mineral by heat, and 

 then, when ground to powder, the substance becomes plaster-of-Paris. 



Alum. In other cases the sulphuric acid, liberated from the decay- 

 ing iron pyrites, attacks the clay itself, and then forms crystals of 

 alum, which is a sulphate of alumina. This product of the London 

 Clay has given its name to Alum Bay in the west of the Isle of 

 Wight ; and it originated the industry of alum-making on the York- 

 shire coast, in beds of the Upper Lias, which are hence called alum 

 shales. 



