ORGANICALLY-FORMED ROCKS 31 
very compact, and forms a rock known as Mudstone, 
or, as it is often called, Claystone. Claystone must 
be ground to powder and moistened before it can be 
moulded like common clay. Clay is often found 
converted into Shale, in which case it has been 
hardened, and can be split up into very fine plates. 
When clay rocks are subjected to great compression 
they may become Slates. Clay mixed with lime 
forms Marl, and sand and clay become Loam. 
I do not propose to write at any length about 
Chemically-formed Rocks, lest I should tax you 
overmuch. As their name implies, they are formed 
or deposited by chemical action, and to understand 
their nature and history you must have a knowledge 
of chemistry. These rocks include some forms of 
limestone, as well as gypsum, rock-salt, ironstone, 
and vein-quartz. They are important to the human 
race on account of their utility in our manufactures. 
Organically-formed Rocks owe their existence to 
plants and animals, whose hard tissues, shells, and 
skeletons, are formed of mineral substances. When 
such organisms perish their durable parts remain, 
become fossils, and accumulate in great quantities 
in seas and lakes. Eventually they form beds of 
rock, sometimes of great thickness. Most of the 
limestones are organically-formed, and such are 
composed of the mineral remains of oysters, whelks, 
mussels, and numerous other shellfish whose shells, 
formed from carbonate of lime extracted from water, 
are their skeletons. Corals have skeletons of car- 
bonate of lime, and the coral reefs which they build 
become consolidated into limestone. Sea-lilies, known 
as Crinoids, are really animals, and their long stems 
