Correspondence—J. W. Evans. — 189 
sea; (2) erosion; and (3) accretion. Since Neolithic man arrived in 
Britain, there is evidence of a submergence along the coasts of England 
and Wales and the Sonth of Ireland, and of an emergence along the 
coasts of Scotland and the North of Ireland. These movements took 
place prior to the time of the Roman occupation of Britain. Whether 
movements have taken place in still more recent times is somewhat 
doubtful, but there are slight indications of a subsidence in the 
extreme North of Scotland, and at one or two places on the coasts of 
England and Wales. Reference was made to the need of systematic 
observations being carried out by the Ordnance Survey to ascertain 
whether such movements are now in progress, and, if so, to what 
extent. 
Estimates were given as to the extent of erosion and accretion in 
the United Kingdom within recent years, and it was shown that 
more land had been gained by accretion and artificial reclamation 
than has been lost by erosion. But while the gain has been almost 
entirely in tidal estuaries, the loss has been on the open coasts. 
Further, the gains have been due not so much to the accumulation of 
material eroded by the sea as to the deposition of sediment brought 
down by rivers from their drainage areas. 
The serious erosion on the coasts of Holderness and of East Anglia 
were described and illustrated by views. While the chief agents of 
erosion are well understood, it was pointed out that there is a great 
lack of knowledge as to what takes place below the level of low water. 
Observations are needed regarding such questions as submarine erosion, 
the travel of material below the low-water line, and the movements 
of outlying sandbanks. There is much obscurity as to the limits of 
depth at which materials are moved on the floor of the submerged 
continental platform by waves of current action, or both combined; 
and again as to what depth the movement of detritus on the sea-floor 
is really effective in producing abrasion. The intermittent character 
of the erosion at many places on the east coast of England was noted 
as a puzzling fact. It may be due to an alteration in the point of 
attack of the sea on the coastline, brought about by the shifting of 
outlying sandbanks or shoals. . 
Another factor in erosion, the importance of which has been over- 
looked, is the action of rock-boring organisms. An account was given 
of the present state of knowledge regarding their work and of their 
effect on the sea-bed near Cromer. Little is known as yet as to the 
rate of boring, or as to the depth to which it occurs. 
CORRESPON DHNCE. 
DIFFERENTIATION IN IGNEOUS ROCKS. 
Sir,—Having occasion to refer to the report of my contribution 
to the discussion ‘‘Sur la différentiation dans les magmas ignés”’ at 
the Toronto Geological Congress, in the Compte Rendu (pp. 248-9), 
I find that through some typographical accident, which in the 
absence of a proof remained uncorrected, the meaning of one 
paragraph has been seriously obscured. 
