306 



PROCEEDINGS O THE GEOLOGICAL SOCIETY. 



Fig. 4. Thorpe Chalk Pit. 

 ( Cross Section of the two furrows G M and A B from G to A. ) 

 S.W. S.E. 



1. Solid chalk. 



o. Yellow ochre. 



u. Umber. 



S. Sand. 



p. rounded pebbles. 



3. reconstructed chalk. 



4. Sand. 



Near the line of section no in each of the furrows, at the 

 height of about a foot from the bottom, was a layer of chalk about 

 4 feet thick, but thicker towards the sides of the furrow than 

 towards the middle. The chalk bore evident marks of reconstruction 

 in some of its parts, but in other parts was apparently so solid as 

 to render it difficult for the observer to believe that it had ever 

 been disturbed. The upper surface of the reconstructed chalk in 

 each of the two furrows appeared to have been exposed to the same 

 kind of furrowing action which the solid chalk has undergone at the 

 base of the furrows. From the top of this layer of reconstructed 

 chalk to the level of the surface of solid chalk, upon this line of sec- 

 tion, was a perpendicular height of 3 or 4 feet. This space in the 

 furrow A b was filled with fine sand, intermixed frequently with 

 much umber, less frequently with yellow ochre, and sometimes 

 with a mixture of the two ; and in one part, where the surface of 

 the layer of reconstructed chalk was depressed below its ordinary 

 level, a layer of rounded gravel occurred beneath the sand. Above 

 the layer of reconstructed chalk, in the furrow A b, small conical 

 protuberances, about 4 inches in length and diameter, projected 

 from the sides of the furrow into the solid chalk, in which they 

 formed cavities, and these cavities were filled with umber. (See 

 fig. 2.) 



It is probably from the decomposition of the iron pyrites in the 

 chalk, that the yellow ochre in these furrows has resulted ; for it is 

 often seen forming small lumps in the solid chalk, near the surface of 



