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we fhall give a fhort account of the principalfa£fs that are ascertained on this fubject.

The independent or true coal-formation con-Gfts effentially of extenfive parallel ftrata ofcoal, covered by ftrata of {hale, which containimpreffions of vegetables, and fometimes theremains of frefh-water fhell-fifh. Beneath eachftratum of coal is generally a ftratum of fome-what greafy indurated clay, called by the minersclunch, which is ufually, if not always, deftituteof /thofe organic remains that charadierize the{hale. It rarely happens that the flate belongingto an inferior ftratum of coal is in immediatecontadt with the clunch of a fuperior ftratum,the coal feams being generally feparated fromeach other by beds of various coloured fand-ftone, of clay, of bituminous {hale, of ratchilor rubble-ftone, of a foft decompofing clayporphyry, or grunftein, called by the Englilhminers rotten-ftone, of argillaceous iron ore,of marl, and of fecondary limeftone. Some-times one, often feveral of thefe ftrata interpofebetween the feams of coal in irregular alter-nation, and there are extenfive collieries inwhich fome of them, efpecially the limeftone, areentirely wanting. The number of feams of coalin any particular formation or coal-field is ex-tremely variable; it feldom however happensthat there are more than three or four worththe expence of working. The uppermoft feamis generally {hattery, and very much mixed withearth and pyrites. The parallelifm of the va-rious ftrata is for the moil part very exactlypreferved, though where a very thick bed offandftone occurs, the ftrata above and below ithave not always the fame parallel. The gene-ral pofition of the beds in the independent coal-formation is fcarcely ever horizontal, and neververtical, though occafionally forming an angleof about 75 0 . The regularity of the dip, how-ever, is frequently interrupted by partial dif-ruptions, flips, and linkings of the ftrata, em-phatically called by the miners troubles , whichappear like great cracks and are filled with fand,gravel, clay, &c. and afford a free paffagefor the water. Sometimes the ftrata are dividedby vertical walls of bafalt, rifing from an un-known depth to the furface, and of vaft length;thefe are called by the miners dykes, and differfrom the flips fti being impermeable to water,and in limply dividing the ftrata without alter-ing their direftion : both the flips and the dykeshowever are obferved to {hatter and pulverize,and decompofe the coal in their immediate vi-cinity. _ The particular fpecies that are foundin the independent formation are flate coal, with

its varieties, cannel coal, and fometimes pitch-coal. Almoft all the Englilh coals, and thofe inthe neighbourhood of Liege , are found in thisformation.

The fecond coal formation is characterizedby the neweft floetz-trap. In this the ftrataare not fo numerous nor fo perfectly parallelas in the former. The coal is not covered with{hale, but with clay or bafalt, in which are foundneither vegetable impreffions nor animal re-mains. The beds which compofe this forma-tion are wakke, bafalt, both amorphous andcolumnar, grunftein porphyry, and argillaceousiron ore. Slips or difruptions of the ftrata arenot common in this formation, but dykes are offrequent occurrence. The fpecies of coal whichit contains are principally pitch-coal, alfo moorand glance coal, and fometimes flate-coal.The chief collieries of Scotland , thofe ofAuvergne and the central parts of France , ofthe Meiffner in Heffe, and the Mittelgebirge inBohemia are examples of this formation.

The third coal-formation occurs in alluvialland, and poffeffes many characters that diftin-guifti it from the preceding ones. The onlyftrata which are found in it, are clay, fand, andgravel. The feams of coal do not preferve thefame thicknefs through the whole of their ex-tent, nor is the parallelifm of the earthy ftrataby any means regular, fudden elevations anddepreffions in the fame ftratum, frequentlytaking place. There are no flips or dykes inthis formation, and every appearance concurs toprove its very recent origin. The fpecies ofcoal that it contains belong chiefly to the brown-coal family, namely bituminized wood, earthyand common brown-coal, moor-coal, and rarelypitch-coal.

The figns of coal are very uncertain both inthe floetz-trap and alluvial formations. In thefirft of thefe no appearance is to be dependedon except the adtual outburft of coal, or of abed of clay, containing fragments of coal. Inthe fecond, the only probable indication is clay,with branches and trunks of treespetrifiedwood is often found in confiderable quantity infandjlone ftrata, without there being the flighteftprobability of a bed of coal beneath.

The independent coal-formation has howevermore numerous figns, and thofe better to bedepended on. Being the oldeft of the threeformations it is fituated nearer the primitiveand tranfition rocks than the others, and in ge-neral appears to rank immediately after fecond-ary limeftone: where beds of this adjoin, andreft upon flate, as is frequently the cafe in the