1398] NOTES AND COMMENTS 83 



ness coast are made up of soft glacial clays, capped in one or two 



places by lacustrine deposits of small extent. They vary in height 

 from 10 to 50 feet, and at Dimlington reach over 100 feet. Mr 

 J. L\ Boyle has shown on historical evidence, and the Rev. E. M. 



<<>le and others have proved by direct observation, that the whole of 

 the cliffs from Bridlington to Spurn are being eroded at an average 

 rate of about 7 feet per annum. The whole of the eroded material 

 must be gradually, or in some cases quickly, converted into gravel, 

 sand, and mud, and carried southwards. A large quantity of this 

 material is carried past the Humber mouth and is gradually silting 

 up in the Wash and off the Lincolnshire coast. At the same time a 

 deal of it must be brought into the Humber at each tide ; and when 

 the winds are the strongest, and the erosion most severe, the inrush 

 of water into the Humber is likewise the greatest. This water brings 

 with it the cliffs in a modified form. It would appear, therefore, 

 that it is from the coast that the bulk of the material suspended in 

 Humber waters is derived. 



It does not follow that the mud now in suspension in the 

 Humber is the result of one or two tides. The particles may have 

 been accumulating during several months, and undoubtedly pass 

 and repass a particular point several times a week. Consequently, 

 when the rivers flowing into the Humber are swollen with flood 

 waters, and are swift, the muddiness observed near their entrances 

 to the estuary is not necessarily due entirely to the additional 

 material which they have brought down, but is more likely to be 

 owing to the sediment in the Humber being stirred up. 



Absolute confirmation of this theory as to the origin of the 

 Humber mud, such as might under other circumstances be afforded 

 by microscopic examination of the mud-particles, is not to be 

 obtained, since the particles brought down by the rivers are 

 precisely similar to those found in the cliffs of the .east coast. 

 Not only are the boulders in those cliffs formed of rocks similar 

 to those eroded by the rivers near their sources ; but in their lower 

 reaches the rivers traverse boulder-clay areas. 



Slugs 



There are before us some papers dealing with slugs, and written by 

 Mr Walter E. Collinge, one of the few British workers who have 

 turned their attention, with any persistency, to this branch of 

 malacology. 



The forms included among slugs are not of necessity near 

 relations, nor are they, as some might imagine, sharply separated 

 off from types with well-developed shells. Nevertheless, slugs as a 

 whole present the same difficulties, and require to be approached 

 from the same point of view. 



