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British Association for the Advancement of Science. 193 
the excavation and filling of the pipe proceeded contemporane- 
ously and gradually, and that the flint nodules, when removed 
from their chalky matrix, subsided so as to rest upon sand and 
gravel which had previously sunk. As proving that the contents 
of the sand pipes came into their present position by slowly. sub- 
siding, the author mentioned the fact of strata of gravel elsewhere 
horizontally bending downwards into the mouth of a pipe, so as 
to become for a short space quite vertical within the pipe. He 
thought that the tubes, or at least some of the larger and deeper 
ones, were caused by springs impregnated with carbonic acid, 
which rose upwards through the chalk. But, afterwards, when 
these springs ceased, the descent of rain water, percolating the 
gravel, carried fine particles of sand and clay downwards, and 
deposited them at the bottom and sides of the tube, at all those 
points where the water was absorbed by the surrounding chalk. 
Some of the finer particles being carried into the chalk itself, 
caused the impurity and discoloration of that rock near the pipes. 
Mr. De la Beche mentioned that similar appearances are observed 
in other formations, as in green sand near Charmouth. Dr. Buck- 
land agreed with Mr. Lyell as to the origin of the clay which 
_ lines these fissures. 'The gravel which covers the chalk he con- 
ceives to have been accumulated under salt water; and after the 
elevation of the strata, so as to become dry land, the clay lining 
was formed by the downward filtration of atmospheric water, 
carrying with it the material in solution. He did not agree with 
Mr. L. in considering these sand pipes as chimneys for the car- 
bonic acid, as he saw no reason why the acid should come up in 
One place more than another, and a place serving as a chimney, 
Should bear the marks of corrosion. Dr. B. concluded by in- 
Stancing the example of fishes killed by carbonic acid, mentioned 
. by Dr. Daubeny. In these, the death of the fish was very sud- 
den, none living above five minutes. In volcanic districts the 
carbonic acid generated must have had a similar effect, and many 
Specimens of fossil fish show the animals to have died suddenly, 
from the perfection of their preservation. He also insisted upon 
the importance of impressions of foot-marks, of atmospheric and 
of watery action on the surfaces of rocks, and stated that the 
Most interesting point now in geology is the examination of those 
Surfaces as they were exposed in different ages. 
