There are many striking examples of this land-building by rivers; and the deltas of rivers, so called from their resemblance to the Greek letter Δ, form in some instances great areas. The Mississippi, the Nile, and the Ganges, for example, are surrounded by great tracts of land at their mouths, which are formed entirely from matter brought down by the rivers and deposited at lower levels than those at which the rivers originated. The Mississippi, which drains a river basin of 1,147,000 square miles, has an annual discharge of sediment of no less than 7,459,267,200 cubic feet. The Italian River Po, draining an area of 30,000 square miles, discharges 1,510,137,000 cubic feet of sediment annually. This is equivalent to a lowering of its whole drainage area by 1/729th of a foot per annum, so that in a thousand years the whole area over which it flows has been lowered by the river by more than a foot. The Thames alone carries down 5,000,000 tons of material each year. All this must be redeposited somewhere. Where the redeposition takes place we find new land forming, new beds, new _strata_, in which in ages to come the future tenants of the globe may find relics of the people and animals living to-day. Thus there are several evident ways in which the coast-line of a country might be altered, either in the direction of enlarging its boundaries by additions to it made by the sea or by rivers; or in the direction of losing parts of its territory by wear and tear. But there are other changes going on which are not so easy to perceive, and which are not so easy to account for. The thing hardest to explain is why what is now dry land should have risen out of the sea, as certainly it did. The white cliffs of Dover are made of chalk, and chalk is made of innumerable shells of tiny animals which once lived in the sea and which at their death sank to the sea's bottom. They steadily accumulated there for ages in a grey ooze, and in course of time this grey ooze rose above the waves. It dried and became land. But chalk is not found in cliffs by the sea only. It is found far inland. It is found, for example, in the North Downs, which run from Guildford to Reigate and from Reigate to Limpsfield and Westerham--a great ridge of chalk, at some points 600 to 800 feet high. That ridge must at one time have been at the sea bottom. And if we were to examine the whole of England and sink borings in it, we should at one point or another come to some remains of rocks, or some "strata," as they are called, which are of such make and material that we can only believe them to have been laid down at the sea bottom. The only conclusion we can come to, therefore, is that by some means or other, and at some time or other, the islands of England were slowly lifted above the sea, and that at some other time the sea was slowly lifted above them. What is true of England is true of nearly all the regions of the world that have been closely examined by geologists.