There are many secondary influences of a less important nature which are due to the ocean streams. The reader will find on most wall-maps of the world certain areas in the central part of the oceans which are noted as Sargassum seas, of which that of the North Atlantic, west and south of the Azore Islands, is one of the most conspicuous. In these tracts, which in extent may almost be compared with the continents, we find great quantities of floating seaweed, the entangled fronds of which often form a mass sufficiently dense to slightly restrain the speed of ships. When the men on the caravels of Columbus entered this tangle, they were alarmed lest they should be unable to escape from its toils. It is a curious fact that these weeds of the sea while floating do not reproduce by spores the structures which answer to the seeds of higher plants, but grow only by budding. It seems certain that they could not maintain their place in the ocean but for the action of the currents which convey the bits rent off from the shores where the plant is truly at home. This vast growth of plant life in the Sargassum basins doubtless contributed considerable and important deposits of sediment to the sea floors beneath the waters which it inhabits. Certain ancient strata, known as the Devonian black shale, occupying the Ohio valley and the neighbouring parts of North America to the east and north of that basin, appear to be accumulations which were made beneath an ancient Sargassum sea. The ocean currents have greatly favoured and in many instances determined the migrations not only of marine forms, but of land creatures as well. Floating timber may bear the eggs and seeds of many forms of life to great distances until the rafts are cast ashore in a realm where, if the conditions favour, the creatures may find a new seat for their life. Seeds of plants incased in their often dense envelopes may, because they float, be independently carried great distances. So it comes about that no sooner does a coral or other island rise above the waters of the sea than it becomes occupied by a varied array of plants. The migrations of people, even down to the time of the voyages which discovered America, have in large measure been controlled by the run of the ocean streams. The tropical set of the waters to the westward helped Columbus on his way, and enabled him to make a journey which but for their assistance could hardly have been accomplished. This same current in the northern part of the Gulf Stream opposed the passage of ships from northern Europe to the westward, and to this day affects the speed with which their voyages are made. THE CIRCUIT OF THE RAIN.