262 ECONOMIC GEOLOGY 
as the deltas of the Ganges and Irrawadi. It is also formed 
in swamps and shallow pools in coastal plains, such as the 
Dismal Swamp of Virginia where it is forming over an area 
of 1000 square miles. Swamp peat may contain but little 
earthy material or ash, because the streams are filtered by 
a fringe of vegetation, and only sediment blown in by the 
wind reaches the middle ; such peat may pass on the margin 
into mud. 
Tropical forests produce beds of decayed vegetation, as 
beneath their canopy of foliage the sodden undergrowth and 
fallen leaves and branches form forest peat. 
Peat in its raw state contains from 20 to 90 per cent. of 
water and usually about 80 per cent.; the amount may be 
reduced to about 20 per cent, by air-drying. Owing to the 
cost of handling and drying, peat is not an economical fuel, 
but being often low in sulphur, may be made into sulphur- 
low briquettes. It often contains from 1 to 2 per cent. of 
nitrogen which may be recovered as ammonia. Moderate 
pressure renders peat dark brown and tough like lignite ; 
pressure alone, even of 6000 atmospheres, has but little 
further effect, unless accompanied by a considerable rise 
of temperature, when peat is made hard and brittle like 
coal. 
LieNITE OR BROWN Coar—Lignite or brown coal is dark 
brown and tough, and often shows woody fibres. It has 
no regular jointing, but splits into layers and weathers with 
curved or flat surfaces. It generally contains from 10 to 
35 per cent. of moisture; a little pyrites is usually present, 
and the amount may be large. Lignite is light in weight 
(sp. gr. up to 1:3). The fixed carbon varies from 15 to 50 
per cent., the volatile constituents from 25 to 50 per cent, 
and the nitrogen is usually about 1 per cent.; when freed 
of ash and moisture its heat-giving value is from 10,000 to 
12,000 B.T.U. Its main defect as fuel is that it readily falls 
to powder; so much is lost in transport, and as unburnt 
powder in the smoke. Lignite is used largely as briquettes, 
especially in Germany, where about 4 cubic yards of lignite, 
which before the War cost from a shilling to six shillings to 
mine, make one ton of briquettes. 
Brown coal deposits are sometimes of enormous thickness. 
A bore at Morwell in Victoria passed through 781 feet of