Читать книгу The Romance of Modern Geology. Describing in simple but exact language the making of the earth with some account of prehistoric animal life онлайн

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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. Everywhere there is the evidence of different stages of existence in the land's history—stages when it was covered by the sea; stages when it was dry land again; perhaps stages when it was covered by lakes, by vast forests; stages when it may have been covered by ice; stages when it was desert. Some of these stages show far vaster upheavals than others, and the changes wrought were of far greater extent. Everybody has heard that the great Saharan desert was perhaps once the bed of an ocean. That is an assertion to which, perhaps, we may be a little chary of committing ourselves; but there is excellent reason for believing that once some of the great African lakes were connected with the sea; and we are quite certain that once Africa was an island. So that in the case of that vast continent we know that it must have seen periods of great depression and elevation; ages when it was much lower than it is now, and ages when it was higher.