It must, however, be observed, that these are what may be termed extreme examples, and that all geologists have not carried the extravagance of their conceptions to such a length as those which we have just cited. Yet, among those who have proceeded with more caution, and have not searched for geological causes beyond the limits of physical and chemical science, much diversity and contradiction still prevail. _Diversities of all the Systems._ According to one system, every thing has been successively precipitated by crystallization, and deposited nearly as it exists at present; but the sea, which covered all, has gradually retired[25]. According to another, the materials of which the mountains consist, are incessantly worn down and carried off by the rivers to be deposited at the bottom of the sea, where they are heated under an enormous pressure, and form strata, which are one day to be violently lifted up by the heat which consolidates them[26]. A third supposes the fluid divided into a multitude of lakes, placed, like the seats of an amphitheatre, above each other, which, after having deposited our shelly strata, have successively broken their dikes, to descend and fill the basin of the ocean[27]. According to a fourth, tides of seven or eight hundred fathoms depth have carried off, from time to time, the matter lying at the bottom of the sea, and have thrown it, in the form of mountains and hills, upon the original valleys or plains of the continent[28]. A fifth makes the various fragments of which the earth is composed, fall successively from heaven, in the manner of meteoric stones, bearing the impress of their foreign origin in the unknown beings whose remains they contain[29]. A sixth represents the globe as hollow, and places within it a loadstone nucleus, which is transported from one pole to the other, by the attraction of comets, carrying along with it the centre of gravity, and the mass of waters at the surface; thus alternately drowning the two hemispheres[30]. We might mention twenty other systems, as different from one another as those enumerated. And to prevent mistake, we may here state, that our intention is not captiously to criticize or find fault with their authors; on the contrary, we admit that these ideas have generally been conceived by men of intellect and knowledge, who were not ignorant of facts, several of whom had even travelled extensively for the purpose of examining them, and who, in this manner, made numerous and important additions to science. _Causes of these differences._