For our purpose swamps may be divided into three groups--climbing bogs, lake bogs, and marine marshes. The first two of these groups depend on the movements of the rain water over the land; the third on the action of the tides. Beginning our account with the first and most exceptional of these groups, we note the following features in their interesting history: Wherever in a humid region, on a gentle slope--say with an inclination not exceeding ten feet to the mile--the soil is possessed by any species of plants whose stems grow closely together, so that from their decayed parts a spongelike mass is produced, we have the conditions which favour the development of climbing bogs. Beginning usually in the shores of a pool, these plants, necessarily of a water-loving species, retain so much moisture in the spongy mass which they form that they gradually extend up the slope. Thus extending the margin of their field, and at the same time thickening the deposit which they form, these plants may build a climbing bog over the surface until steeps are attained where the inclination is so great that the necessary amount of water can not be held in the spongy mass, or where, even if so held, the whole coating will in time slip down in the manner of an avalanche. The greater part of the climbing bogs of the world are limited to the moist and cool regions of high latitudes, where species of moss belonging to the genus _Sphagnum_ plentifully flourish. These plants can only grow where they are continuously supplied with a bath of water about their roots. They develop in lake bogs as far south as Mexico, but in the climbing form they are hardly traceable south of New England, and are nowhere extensively developed within the limits of the United States. In more northern parts of this continent, and in northwestern Europe, particularly in the moist climate of Ireland, climbing bogs occupy great areas, and hold up their lakes of interstitially contained water over the slopes of hills, where the surface rises at the rate of thirty feet or more to the mile. So long as the deposit of decayed vegetable matter which has accumulated in this manner is thin, therefore everywhere penetrated by the fibrous roots of the moss, it may continue to cling to its sloping bed; but when it attains a considerable thickness, and the roots in the lower part decay, the pulpy mass, water-laden in some time of heavy rain, break away in a vast torrent of thick, black mud, which may inundate the lower lands, causing widespread destruction.