THERE is scarcely any level land around Santo Domingo, but in every direction a succession of hills and valleys. The hills are not isolated; they run in irregular ranges, having mostly an east and west direction, but with many modifications in their trend. From the main valleys numerous auxiliary ones cut deeply into the ranges, and bifurcate again and again, like the branches of a tree, forming channels for carrying off the great quantity of water that falls in these rainy forests. The branching valleys, all leading into main ones, and these into the rivers, have been excavated by subaerial agency, and almost entirely by the action of running water. It is the system that best effects the drainage of the country, and has been caused by that drainage. The wearing out of valleys near Santo Domingo proceeds more rapidly than in regions where less rain falls, and where the rocks are not so soft and decomposed. Even during the few years I was in Nicaragua there were some modifications of the surface effected; I saw the commencement of new valleys, and the widening and lengthening of others, caused not only by the gradual denudation of the surface, but by landslips, some of which occur every wet season. The rocks of the district are dolerytes, with bands and protrusions of hard greenstones. The decomposition of the dolerytes is very great, and extends from the tops of the hills to a depth (as proved in the mines), of at least two hundred feet. Next the surface they are often as soft as alluvial clay, and may be cut with a spade. This decomposition of the rocks near the surface prevails in many parts of tropical America, and is principally, if not always, confined to the forest regions. It has been ascribed, and probably with reason, to the percolation through the rocks of rain-water charged with a little acid from the decomposing vegetation. If this be so, the great depth to which it has reached tells of the immense antiquity of the forests.