Page:The Journal of geology (IA journalofgeology21894univers).pdf/255

high that the course of drainage was altered. The new valleys were cut in Galena limestone. The amount of cutting in the limestone since maximum glaciation is about equal to that in the newer drift in the vicinity of Lake Michigan. The later work has been going on, it is believed, about 7000 years. It is estimated that erosion in the limestone of northwestern Illinois took place one tenth as fast as in the drift. On this basis 70,000 years have been required for the erosion accomplished in northwestern Illinois since the maximum period of glaciation. Long after the withdrawal of the maximum ice-sheet, a mantle of loess was spread over northwestern Illinois. The writer thinks that something like four fifths of the erosion accomplished since the withdrawal of the maximum ice-sheet was accomplished before the deposition of the loess. Fifty thousand years are considered a minimum, and perhaps twice that time not too great an allowance of time, for the erosion that took place between the time of the formation of the drift sheet in northwestern Illinois and the deposition of the loess.

These notes are descriptive of the dikes at the Helder and at Petten in North Holland, and at West Kappele in Zealand. The breaks in the coastal dune ranges are occupied by them. The whole sea-front is protected by a system of jetties also. They are built on the strand and in front of the dikes—and check the currents which carry away the beach sands and tend to undermine the dune hills at these localities. The dikes are essentially enormously thick walls of sand whose outer slope is at a low angle, and is faced with stone and further protected by rip-rap and by piling. The descriptive notes of the construction are illustrated with plans and vertical cross-sections.

The application of a modified system of sea-dikes for the protection of the bluffs at Long Branch, New Jersey, follows. The reclamation of the tidal lands of the state is referred to, and the reclamation of the low-lands of Holland affords an instructive example.