ROCKS, THEIR KINDS, STRUCTURE, AND DISPOSITION. 137 
because it cleaves or sticks; and the whole accumulation of mud, clay, 
and sand at the bottom of any water, is called silt, 
The remains of vegetable matter found in various parts of the country, 
_ and used as fuel, are known as peat; and coal is nothing but such vege- 
table matter changed by heat, and hardened into rock by pressure. Lime- 
stone is the name given to the hard rock which, after being burned in a 
kiln, forms lime. When the limestone is hard and crystalline, it forms 
marble, which is of different colours, from deep black to pure white, and 
often beautifully variegated. Chalk is a variety of limestone, and obtains 
its name from this fact ; the word chalk being another form of the Latin 
calx, lime. 
Common slate, used for writing on and for roofing, is composed of 
thin layers of hard rock, of which some of our highest mountains 
are formed. The name shale is applied to a kind of rock which shells 
off or splits into very thin layers, and which may be seen in great heaps 
near coal-pits. Thin layers of sandstone used for pavement are called 
flags. The white pebbles so common on the sea-beach, and so easily 
broken, are made of quartz, and rock formed of it is called quartz-rock. 
Some varieties of quartz, called rock-crystals, are very beautiful and 
valuable, and become even precious stones, such as agate, amethyst, and 
topaz. lint has much the same composition as quartz, and is very 
plentiful in chalk. The granular rock brought from Aberdeen and 
elsewhere, so beautiful when polished, is called granite, from its being 
‘composed of grains of other rocks. The particles that glitter like silver in 
the granite are pieces of mica,! which is so named because it shines. A 
mineral very like mica in appearance, but different in composition, is 
called talc, from its feeling somewhat greasy or tallowy when touched. 
_ The molten matter that flows from volcanoes is called lava;? pumice- 
stone*® is the cinder of such discharges ; while the ashes that are thrown 
into the air are called scorie. In geologic times also there existed 
voleanoes from which lava issued; the rock this lava formed is called 
trap,* from lying in stair-like masses, as it flowed from the mountain ; 
and one kind, whinstone, which is much used for roads. A common 
variety is known as greenstone, from its colour, of which Salisbury 
Crags, near Edinburgh, are composed. Another variety is called 
basalt, and is generally found in columns standing close together, 
which often form wonderful natural scenes, such as Fingal’s Cave 
and the Giant’s Causeway. Another variety is porphyry,5 so called from 
its frequent purple colour, and is easily distinguished by its granular 
appearance. A kind of light porous rock, formed of cohering volcanic 
1 From Latin mico, to shine, 4 From Latin trappa, a stair. 
2 From Latin Javo, to flow. > From Latin porphyra, purple. 
3 From Latin pumez, -icis, cinder, 
