But the great lake with all its varied stores was doomed to sink lower and lower, till the great sea overwhelmed England. Another ocean joining it to the east overwhelmed Germany; and the whole of Europe, south of a line drawn through Scotland, Christiania, and Moscow, became sunk under salt water. There were patches standing up here and there--Ireland, Brittany, Cornwall, Spain or a good part of it, Switzerland, part of Italy (and also part of what is now the Western Mediterranean), and most of Turkey and Hungary. But elsewhere marine animals succeeded the reptiles, and the foundations of all the chalk hills and cliffs of modern Europe were laid. Of what were they made? We may borrow a capital suggestion from Mr. Jerome Harrison, of Birmingham University. "Take," he says, "a piece of chalk and brush it vigorously with a tooth-brush in a glass of water until the liquid looks quite milky. Allow the greater part of the sediment to subside, and then pour away the water and wash the material which has sunk to the bottom of the glass by pouring water on it two or three times. Put the whitish powder which finally remains under a microscope; and examine it with, say, the quarter-inch power, which will magnify about 300 diameters. The greater part of the white powder will then be seen to be composed of the minute shells of creatures called Foraminifera--little specks of jelly-like matter which secrete for themselves a shell or covering from the carbonate of lime dissolved in the sea-water in which they live. "Countless millions of foraminifera inhabit the waters of the North Atlantic (and of other deep seas) at the present day; and of these at least one species--_Globigerina bulloides_--cannot be distinguished from one of the commonest species found in the White Chalk. When these tiny animals die their soft parts soon decay and disappear, and their skeletons (or shells) fall on the sea floor, where they form a whitish mud or 'ooze.' The time required for the accumulation of so thick a deposit composed of the remains of organised beings--the White Chalk is in Norfolk quite 1200 feet thick--must have been very great. If we allow that the tiny shells of the foraminifera may have accumulated at the rate of two feet in thickness in a century, then it would have required 50,000 years to form the chalk of the south-east of England, whose thickness we have estimated at 1000 feet."