TRIMMER ON SAND-GALLS IN THE NORFOLK CHALK. 301 



intitled "On Pipes or Sand-galls in Chalk*," I have shown that, - 

 in the county of Kent, these cavities in the chalk have, in their 

 upper part, a longitudinal extension, and are connected with fur- 

 rows which traverse the upper surface of the chalk, and which 

 widen and deepen as they approach the pipes. From the nature 

 of the materials with which these cavities are filled, I inferred 

 that, in that county, they were formed before the deposit of the 

 oldest Eocene strata. From the fact, also noticed by me in the 

 same county, that similar furrows and pipes, though of smaller 

 dimensions, occur in the blocks of siliceous sandstone which are 

 dispersed through the superficial deposits, and are derived from 

 the sands of the plastic clay, I concluded that these cavities were 

 caused, not so much by the chemical as by the mechanical action 

 of water ; and, from the further observations I made, that still 

 smaller furrows and pipes are actually in the course of formation 

 on the surface of similar siliceous blocks on the coast of Kent, I 

 judged that the mode of this mechanical action was by the flux 

 and reflux of waves breaking on the shore. 



Having now extended my observations to the chalk in the vici- 

 nity of Norwich, I find that in that district also the pipes consti- 

 tute the termination of longitudinal furrows ; and in this paper I 

 shall adduce evidence, from the phenomena observable in the 

 superincumbent strata of sand and gravel, that these hollows were 

 excavated in the chalk just before the deposit of the Norwich Crag, 

 and that, like the pipes in the chalk of Kent, they owe their origin 

 to the mechanical action of water. 



I will first offer some general remarks on the chalk, and on the 

 strata overlying the chalk, in the vicinity of Norwich. 



The chalk which bounds the continuous valley watered by the 

 rivers "Wensum and Yare, rises to the height of from 60 to 80 feet 

 above the river level. It is covered by the sands and gravel be- 

 longing to the Norwich Crag and the Northern Drift. These 

 form a nearly level plateau, intersected by many inosculating val- 

 leys, some containing streams, others dry. To judge from the 

 sections laid open around Norwich, the greatest thickness of the 

 supra- cretaceous deposits, at the highest points of this plateau, is 

 about 60 feet : towards the valleys these deposits thin off. 



Immediately upon the chalk rests a ferruginous breccia, locally 

 termed " the pan," from one to two feet thick, and composed of 

 large unabraded or slightly water-worn flints ; and these are some- 

 times mixed with marine shells, unbroken and in fragments. It 

 is in this, the most constant member of the crag series in this part 

 of Norfolk, that the bones of terrestrial mammalia are found. | 



Above the breccia are beds of sand and silt, containing, in some 

 places, considerable accumulations of unbroken marine shells. 

 These accumulations sometimes occur in drifted masses from one 

 to four feet thick ; sometimes they consist of groups of two or three 



* Vide " Proceedings," vol. iv. p. 6. 



+ Mr. Wigham, who has been in the habit of purchasing bones of the work- 

 men for many years, informs me that each pit yields about one bone a year. 



