Independent Coal Formation. >#3 cafes troughs of great extent, or fkirting the lower part of the ranges of mountains, and fometimes to a confiderable diftance. II. It appears to have been univerfally depo sited, becaufe it has been obferved in the mod widely diftant parts of the globe. It differs, how ever, from the greater number of formations in this refpeft, that the individual depofitions of which it is compofed, do not appear to have a ve ry great extent, and are not immediately connect ed together. Thefe individual depofitions, in fome countries extend many miles, in other coun tries they are entirely wanting j but in no inflance do they appear fo extenfive as the floetz limeftone, and other floetz rocks. Hence, it is more than probable, that this broken or interrupted diftri- bution is the original one. This fuppofition is further ftrengthened, by the ufual occurrence of this formation in hollows, and in detached portions, in protected hollows, when it occurs at the foot of mountain groupes ; and by its containing delicate frefh-water ftiells, and im- preffions of ferns in a ftate of perfed prefervation, which would fcarcely have been the cafe if the for mation had been extenfively depofited. 12. Clay-ironftone occurs very abundantly in the ' oldeft formation. Cinnabar, and other ores of mercury, occur in great quantity in certain coal formations, as at ldria and Almaden. Thefe ores are contained in a folid, quartzy, bituminous fand- ftone,