TRIMMER ON SAND-GALLS IN THE NORFOLK CHALK. 311 



Fig. 9. 



1. Solid Chalk. 



2. Sand, gravel, and clay, 



a. Pot in the chalk. 



b. Core above the pot. 



down towards the core ; on the other, they abut abruptly, in a 

 horizontal position, against it. 



Over the larger sandpipes in this pit generally, the stratification 

 of the lower beds of crag is disturbed, and, in some places, quite 

 obliterated. 



From the state of the strata of the uncallow above, the work- 

 men profess to be able to determine the nature of the cavity in 

 the chalk below. It may be stated, as a general rule, that over 

 " drops," or other irregularities in the surface of the chalk, of 

 greater width than depth, the alternating beds of sand and clay 

 are disposed in gently curving flexures, which are evidently ori- 

 ginal conditions of deposit : that over deep pipes, not exceeding a 

 foot in diameter, the laminae of the sand and gravel are undis- 

 turbed ; that over pipes of greater diameter the laminae usually 

 suffer disturbance ; but that even over the widest pipes, the dis- 

 turbance does not extend to the height of 20 feet from the surface 

 of the chalk. When the Uncallow exceeds that thickness, the 

 disturbed strata are overlaid by others which are horizontal. 



Among the contents of the sand-pipes in the solid chalk near 

 Norwich, I have not met with any fragments of chalk ; though in 

 two instances I have seen rounded pebbles of chalk, lying near the 

 clay which usually lines the sides of these cavities. The flints in 

 the pipes are rarely waterworn, and appear to have been rarely 

 detached from the chalk. In Kent, on the contrary (particularly 

 in a pit near Canterbury, described by me in a former communi- 

 cation) a large portion of the flints in the pipes have undergone 

 considerable attrition ; but the form of the pebbles is not orbi- 

 cular nor ellipsoidal, but rather that of two cones, placed base to 

 base. 



As a general fact, crag-shells have not been found in the pipes 

 in the solid chalk near Norwich, even in cases where such shells 

 are abundant in the overlying crag strata. Neither have I dis- 



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