_Second_, That in certain parts of the world this outer crust of rocks is broken through by living volcanoes, which, with intermissions more or less frequent, belch forth flames and smoke, with occasional torrents of burning lava. That where, or when, the cause of such eruptions is not sufficiently powerful to produce living volcanoes, earthquakes are occasioned; cracks or fissures, more or less wide, are produced in the solid rocks; smoking fumeroles appear; and vapour-exhaling surfaces show that fires, though languid and dormant for the time, still exist beneath. That besides the rocks of lava they have poured out, these volcanic agencies change the surface of a country more widely still by the alterations they gradually effect upon the previously existing slaty, calcareous, or sandstone rocks; converting limestone into marble, and baking sandstone into more or less homogeneous quartz, and common slates or hardened clays into mica slates, gneiss, and granite-like rocks. That such volcanic agencies, producing similar phenomena, have existed in every geological epoch; and though the evidences of these are most extensive and distinct, perhaps, among the rocks of the oldest or primary period, that they are numerous and manifest also among those of the secondary and tertiary periods. _Third_, That rocks of every age and kind, when exposed to the action of the air, the vicissitudes of the seasons, the beating of the rains, the force of flowing water, the dash of the inconstant sea, and other natural agencies, crumble down, wear away, or are torn asunder into fragments of every size. These either remain where they are formed, or are carried by winds and moving waters to distances, sometimes very great, but which are dependent on the force of the wind or water which impel them, and on the size or density of the fragments themselves. Thus are our shores daily worn away by the action of the sea, and the fragments distributed along its bottom by the tides and currents; and thus, from the far northern mountains of America, does the Missouri bring down detached fragments thousands of miles into the Gulf of Mexico, whence the Gulf Stream carries them even to the icy Spitzbergen.