552 
THE CRETACEOUS ROCKS OF BRITAIN. 
residue” exceed '06 per cent., except in the Beer Stone and at 
Hitchin, where the samples contain silicihed shell-fragments. 
In the Terebratulina zone the coarse residues examined by him 
average about ’013 per cent., except where a sample happens to 
include a lump of iron-oxide, as in that from Hessle (Yorkshire). 
The average percentages of fine insoluble residue for these zones 
are : — 
In zone of Rh. Cuvieri, *02 per cent. 
In the Terebratulina zone, *013 per cent. j 
Of the deposits explored by the Challenger Expedition and 
Ascribed by Sir J. Murray those which come nearest to such chalk 
are the Pteropod and Globigerina oozes between Tristan da Cunha 
and Ascension Island in the South Atlantic ; these came from 
depths of between 1,415 and 2,025 fathoms, and the proportion 
of mineral debris was in no case more than 1 per cent. Elimi¬ 
nating two cases in which the quantity of deposit obtained was 
too small for reliable results, the remaining seven furnish the 
following average composition : — 
Proportion of 
Proportion 
Average size 
Proportion of 
Calcareous 
of Mineral 
of Mineral 
Fine 
Matter. 
Particles. 
Particles. 
Washings. 
92-66 
0-93 
0-06 
5*42 
The proportion of fine washings varied from 2 to 12 per cent., 
but generally included debris of siliceous organisms which, being 
soluble, would be eliminated from the rock mass when the deposit 
was raised above the sea level. Consequently if this ooze were 
ever to become consolidated as a terrestrial rock, the proportion of 
chalky matter would be raised to 96 or 97 per cent. 
The quantity of included mineral particles varies in different 
places and is naturally great in the neighbourhood of active vol¬ 
canoes or where the prevalent winds blow off tracts of sandy 
desert. In most of the other Pteropod oozes the proportion of 
mineral particles was only 1 or 2 per cent., but the fine washings 
average 19 per cent. It is interesting to note the entire absence 
from the chalk of particles of volcanic eject amenta, and we may 
safely infer that there were no volcanoes on the land areas adja¬ 
cent to the Anglo-Parisian Sea. 
With regard to the size of the mineral particles in the Middle 
Chalk, Mr. Hill finds that in ten samples of the Rh. Cuvieri zone 
the average size is only • 13 mm. though occasional fragments as 
large as *4 or *5 mm. occur. In the Terebratulina zone they 
seldom exceed * 3 mm., and the average size is only ■ 09 mm. 
