ais VOLCANIC ROCKS. It is clay or flate-clay which has experienced greater or lefs degrees of hear, but never fo much as the following fpecies, the porcelain-jafper. 2. Porcelain-jafper.—I his fubftance is particu larly defcribed in the firft volume of this work. It is llate-clay or common clay, which has been expofed to a heat that has converted it into a kind of porcelain. Like the preceding fpecies, it fome- times contains imprefiions of plants; a fadt which fhews that it has not been completely melted. It is evident, aifo, from what has been faid, that both fpecies are in their original repofitory. 3- Parth-Jlag.—As the foflils which are con verted into earth-flag by volcanic fire, differ very much from one another, it is evident that the ap pearance of the flag mull alfo be very various. It very much refembles common forge-flag.' It is black, lometimes brownilh or reddifh, and is fre quently covered with a tempered-fteel tarnifh. It is amorphous or veficular, and hasfometimes a metal lic lufire. Some varieties have an earthy afpeft, but that is rarely the cafe. It contains no foflils wrap ped up, nor are large mafies fo uniform as lava. It originates, probably, from clay or clay-ironflone. 4. Columnar clay-ironjlone. It is ufuully femi- vitrified. 5. Polijhing-jlate. This fubfiahce is defcribed in the firft volume of this work. Werner fufpedts it to be the afhcs of burnt coal, which have been carried by