260 Mr. Lyell on the tubular Cavities Jillcd iioith Gravel 



For the distance of several inches, or even in some places 

 four or five feet from its junction with the sand-pipe, the chalk 

 at Eaton is moist and softened, and becomes friable when 

 dried, and is discoloured by containing a slight mixture of 

 fine sand, clay, and iron, the same chalk being quite pure, and 

 perfectly soluble in acids at points more remote from the 

 pipes. In some cracks and interstices of the chalk, even at a 

 distance from the sand-pipes, are found thin leaf-like layers 

 of reddish and greenish clay, which may have been intro- 

 duced from above through numerous joints which traverse 

 the rock, in a nearly vertical direction, and by which the 

 flints also are sometimes divided. The surface of the flints 

 thus naturally split is discoloured and iron-stained, and distin- 

 guishable from that obtained by fresh fracture. At Eaton 

 the joints do not appear to be connected with the sand-pipes, 

 except here and there accidentally. They have in general a 

 more oblique direction than the pipes. 



The course of a sand-pipe is usually traceable above the 

 level of the chalk for some distance upwards through the in- 

 cumbent gravel and sand by the obliteration of all signs of 

 stratification. In some instances however I observed at the 

 mouth or upper extremity of the pipe, as in the pipe d, fig. 1, 

 beds of gravel and sand bending downwards, so as to attain 

 a perfectly vertical position within the pipe, precisely as 

 would have happened if horizontal beds had subsided, in 

 consequence of a failure of support from below. 



Age of the Gravel. — As to the age of the gravel and sand 

 overlying the chalk at Eaton, there can be no doubt that it 

 belongs to the Norwich crag, as there are not only casts of 

 marine testacea characteristic of that formation in the ferru- 

 ginous sandstone at Eaton, but also, as I learn from Mr. J. B. 

 Wigham, some shells of the genera Mya, Mactra, Cardium, 

 and Mytilus, in which the calcareous matter is still pre- 

 served. 



I am also indebted to Mr. Wigham for the following ob- 

 servations. " At Heigham, in the suburbs of Norwich, are 

 sand-pipes resembling those at Eaton, except that they de- 

 scend in a slanting and often winding course. In the pit 

 represented in fig. 2, which is 30 feet deep, the chalk is 

 barely covered by vegetable soil. Its upper portion to the 

 depth of 4 feet is intermixed with sand and gravel. In the 

 undisturbed chalk below are some irregular cavities, «, b, 

 from 10 inches to 2 feet in diameter, which have no communi- 

 cation with the surface, and which on examination are found 

 to terminate after penetrating horizontally about 2 feet, the 

 chalk in contact being everywhere solid. They are evi- 



