British Association for the Advancement of Science. 123 



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 eflect, 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. 



