112 B. MORTIMER ON A WELL-SECTION IN THE CHALK AT DRIFFIELD. 



drying becomes more friable, and frequently breaks up into scaly 

 pieces, which warp and curve. It seems to be composed of com- 

 minuted chalk and shells mixed with decayed animal and vegetable 

 reman] s ; in short it consists, for the most part, of the wreck of 

 oceanic life ; and a comparison of the two chemical analyses given 

 below, which were made by a well-known analytical chemist (Mr. 

 Thomas Hodgson, of Under Cliff, Bradford), strongly supports this 

 belief : — 



Analysis of sample of chalk from the Analysis of sample of fuller's earth 



well-section at Driffield : — from the well-section at Driffield: — 



Moisture 5-20 Moisture 1215 



Carbonate of lime 93 30 Organic matter and water com- 



Carbonate of magnesia -15 bined 1-67 



Oxide of iron and alumina . . . *20 Carbonate of lime 33*40 



Silica 1*15 Carbonate of magnesia *50 



Oxide of iron 7*05 



100-00 Alumina 1023 



Alkaline salts -20 



Silica 34-80 



10000 



The author adduces the facts above alluded to in support of his hypo- 

 thesis that the solid chalk is built up by organisms of a zoophytic 

 nature, and regards the clayey layers as caused by temporary disturb- 

 ances of the water by which the ordinary quiet " secretion " of the 

 chalk was interrupted. 



Discussion. 



Prof. Hughes thought that the origin of the particular structure 

 referred to by Mr. Mortimer was a question for mineralogists and 

 chemists, though somewhat similar forms might be produced me- 

 chanically. He pointed out that its occasional occurrence along 

 fissures not in the line of bedding was quite conclusive against its 

 being in any way referable to the period of original deposition. As 

 to the more clayey bands, he considered the Chalk, though often com- 

 posed in a great measure of comminuted organisms, to be all sedimen- 

 tary, and had therefore no difficulty in explaining how more clayey 

 matter might accumulate over various areas at different times. 



Mr. Wiltshire mentioned that argillaceous bands extending over 

 wide areas are to be observed in the Chalk. 



Mr. "Whitae^r stated that clay bands occur near Beachy Head. 

 He considered that the argillaceous bands are not seen in hard 

 chalk. The analyses given by the author were of much value. 



Mr. Charles worth remarked that the Yorkshire Chalk presents 

 exceptional characters. The sponges found fossilized in it are sili- 

 cified throughout, but the silica is confined to the sponges, and does 

 not invest them. 



Mr. Koch stated that near Nice the Dolomitic Limestone some- 

 times exhibits a structure similar to that described in the paper, and 

 that when this structure occurs minute crystals can be detected in 

 the mass. 



