ANALYTICAL ABSTRACTS. 24 1 



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 north- 

 western Illinois and the deposition of the loess. J. A. B. 



Notes on the Sea-Dikes of the Netherlands. By Prof. J. C. Smock. 

 (Annual Report of the State Geologist of New Jersey, 1892, pp. 

 315-329)- 



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 sys- 

 tem 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 under- 

 mine 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 Hol- 

 land affords an instructive example. 



J. C. S. 



