Although we cannot obtain a precise knowledge of the action exerted by water within its own bosom, it is yet possible to determine its limits to a certain degree. Lakes, pools, marshes, and sea-ports, into which rivulets discharge their waters, more especially when these descend from near and steep hills, deposit large quantities of mud, which would at length fill them up entirely, if care were not taken to clean them out. The sea also throws quantities of slime and sediment into harbours and creeks; into all places, in short, where its waters are more tranquil than ordinary. The currents also heap up at their meeting, or throw out at their sides, the sand which they are continually raising from the bottom of the sea, forming it into banks and shallows. _Stalactites._ Certain waters, after dissolving calcareous substances by means of the superabundant carbonic acid with which they are impregnated, allow these substances to crystallize after the acid has evaporated; and, in this manner, form stalactites, and other concretions. There are strata, confusedly crystallized in fresh water, which are sufficiently extensive to be compared with some of those which have been deposited by the ancient sea. The famous Travertine quarries of the neighbourhood of Rome, and the rocks of the same substance, which are formed, and continually varied in figure, by the river of Teverona, are generally known. These two modes of action may be combined; the deposits accumulated by the sea may be solidified by stalactite. Thus, when springs abounding in calcareous matter, or containing some other substance in solution, happen to fall into places where these deposits are formed, we then find aggregates in which marine and fresh-water productions may be blended. Of this description are the banks in the island of Guadeloupe, which, along with human skeletons, present land and sea shells mingled together. Of the same nature also is the quarry described by Saussure, in the neighbourhood of Messina, in which the sandstone is seen forming by the consolidation of the sand thrown up by the sea. _Lithophytes._ In the torrid zone, where lithophytes of many species abound, and are propagated with great rapidity, their strong trunks are interwoven and accumulated so as to form rocks and reefs; and rising even to the surface of the water, shut up the entrance of harbours, and lay frightful snares for navigators. The sea, throwing up sand and mud upon the tops of these shoals, sometimes raises their surface above its own level, and forms islands, which are soon covered with a rich vegetation. _Incrustation._