
160 | GEOGNOSY. -[Boox II, 
been here and there changed into gneiss and even into rocks which 
graduate into granite. 
The more fissile fine-grained varieties of this rock have been termed 
greywacke-slate. In these, as well as in greywacke, organic — 
remains occur among the Silurian and Devonian formations. Some- 
times in the Lower Silurian rocks of Scotland these strata become 
black with carbonaceous matter, among which vast numbers of 
graptolites may be observed. 
2. Clay Rocks (Pelites). 
These are composed of the finer argillaceous sediments or mud 
derived from the waste of rocks. Perfectly pure clay or kaolin, 
hydrated silicate of alumina (silica 47-05, alumina 39°21, water 
13-74), may be seen where granites and other felspar-bearing rocks 
decompose. But, as a rule, the argillaceous materials are mixed 
with various impurities. 
Clay, Mud.—The decomposition of felspars and allied minerals 
gives rise to the formation of hydrous aluminous silicates, which 
occurring usually in a state of very fine subdivision, are capable of 
being held in suspension in water, and of being transported to great 
distances. ‘These substances differing much in composition, are 
embraced under the general term Clay, which may be defined as a 
white, grey, brown, red, or bluish substance, which when dry is soft 
and friable, adheres to the tongue, and shaken in water makes it 
mechanically turbid; when moist is plastic, when mixed with much 
water becomes mud. It is evident that a wide range is possible for 
varieties of this substance. The following are the more important, 
Pipe-clay.— White, nearly pure, and free from iron. 
Fire-clay.—A deposit largely found in connexion with coal-seams, 
contains little iron, and is nearly free from lime and alkalies. Some 
of the most typical fire-clays are those long used at Stourbridge, 
Worcestershire, for the manutacture of pottery. The best glass-house 
pot-clay, that is, the most refractory, and therefore used for the con- 
struction of pots which have to stand the intense heat of a glass-house, 
has the following composition :—silica, 73°82; alumina, 15°88; 
protoxide of iron, 2°95; lime, trace; magnesia, trace; alkalies, :90; 
sulphuric acid, trace; chlorine, trace; water, 645; specific gravity, 
2°51. 
Gannister.—A very siliceous close-grained variety, found in the 
Lower Coal-measures of the north of England, and now largely ground 
down as a material for the hearths of iron furnaces. 
Brick-clay.—Properly rather an industrial than a geological term, 
since it is applied to any clay, loam, or earth, from which bricks or 
coarse pottery are made. It is an impure clay, containing a good deal 
of iron, with other ingredients. Ananalysis gave the following com- 
position of a brick-clay : silica, 49:44; alumina, 34:26; sesquioxide 
of iron, 7°74; lime, 1°48; magnesia, 5:14; water, 1°94, 
