
ROCKS 63 
are required to stand intense heat. Arick-clay is an intimate admixture 
of clay and sand with some iron-oxide, and is used for ordinary bricks. 
Fullers earth is a soft, dirty greenish, brownish, blue, yellow, or grey 
variety of clay, somewhat greasy or unctuous to the feel, which falls into 
powder in water. Sha/e is the name given to any argillaceous rock that 
divides into thin layers or lamine, corresponding to planes of deposition. 
Shales vary greatly in composition, some containing much sand (avena- 
ceous shale), others being largely carbonaceous (carbonaceous shale), or 
saturated with bituminous matter (02/7 shale). Alum shale is an argil- 
laceous rock charged with a considerable quantity of disseminated 
pyrite or marcasite (sulphides of iron), through the decomposition of 
which alum (sulphate of alumina) and copperas or iron vitriol (hydrous 
sulphate of iron) are formed. 
Loam is a mixture of sand and clay, usually containing 
some calcium-carbonate, the sand being plentiful enough to 
allow the percolation of water through the mass. Most loams 
are of alluvial origin, and are, therefore, commonly developed 
in valley-bottoms. 
3. GLACIAL ROCKS 
These rocks are the result of the mechanical action of ice. 
Rock-rubble, already described as a subaérial formation, might 
perhaps be classed as a glacial rock, since it owes its origin 
chiefly to the action of frost. It is preferable, however, to 
include here only those formations which are the products 
of glacial erosion and transport. Amongst these, by far the 
most important is Boulder-clay or Till, a more or less 
tenaceous, gritty clay, crowded with angular and subangular 
stones and boulders. It varies, however, very much in 
character, being occasionally more aranaceous than argil- 
laceous. When the larger stones are removed it is often 
used for brick-making; in many places, however, it is too 
stony for such a purpose. 
When subjected to mechanical analysis, the plastic materials of the till 
of the Scottish lowlands is seen to be a heterogeneous aggregate of 
minutely triturated mineral matter, and much rock-flour of a very fine 
consistency. Only a meagre proportion of this so-called “clay” consists 
of hydrous silicate of alumina, or pure clay. Boulder-clay, in short, is 
composed for the most part of unweathered rock-material—it is the result 
of glacial grinding, and has not, like ordinary alluvial clay, been formed 
by the chemical decomposition of minerals and rocks. 
' The only other glacial accumulation that need be referred to are the 
mounds and sheets of earthy rock-débris and boulders which occur in and 
