1898] 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. R. Boyle has shown on historical evidence, and the Rev. E. M. 
Cole 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 perannum. The whole of the eroded material 
must be gradually, or in some cases quickly, converted into gravel, 
sand, and mud, and earried 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. 
