90. If, from the confines of this nucleus, we conceive certain fissures and openings to traverse the solid crust, and to issue at the surface of the earth, the vapours ascending through these may in time heat the sides of the tubes through which they pass to a vast distance from the lower extremities. It is, indeed, difficult to fix the limit to which this distance may extend, on account of the great difference between the rate at which heat moves when it has a fluid for its vehicle, and when it is left to make its way alone through a solid body. In the present case, the supply of heat is rapid, as being made by a vapour ascending through a tube of solid rock; and the dissipation of it slow, as arising from its transmission through the rock. The waste of heat is therefore small, compared with the supply, and grows smaller at every given point, the longer the stream of heated vapour has continued to flow. Such a stream, therefore, though it may at first be condensed within a small distance of its source, will in time reach higher and higher, and may at last be able to carry its heat to an immense distance from the place of its original derivation. Thus, it is easy to conceive, that vapours from the mineral regions may convey their heat to reservoirs of water near the surface of the earth, and may in that manner produce hot springs, and even boiling fountains, like those of Rycum and Geyser. 91. When, instead of a heated vapour, melted matter is thrown up through the _shafts_ or _tubes_, which thus communicate with the mineral regions, veins of whinstone and basaltes are formed in the interior of the earth. When the melted matter reaches to the surface, it is thrown out in the form of lava, and all the other phenomena of volcanoes are produced. Lastly, where melted matter of this kind, or vapours without being condensed, have their progress obstructed, those dreadful concussions are produced, which seem to threaten the existence even of the earth itself. Though terrible, therefore, to the present inhabitants of the globe, the earthquake has its place in the great system of geological operations, and is part of a series of events, essential, as will more clearly appear hereafter, to the general order, and to the preservation of the whole. Such, according to this theory, are the changes which have befallen mineral substances in the bowels of the earth; and though different for the stratified and unstratified parts of those substances, they are connected together by the same _principle_, or explained by the same _cause_. It remains to consider that part of the history of both which describes their changes after their elevation to the surface; and here we shall find new causes introduced, which are more directly the subjects of observation, than those hitherto treated of; causes, also, which act on all fossils alike, and alike prepare them for their ultimate destination. SECTION III.