AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 89 
above O.D. The plateau is dissected by wide valleys with gently sloping sides and 
flat alluvial bottoms excavated in the Upper Lias clays. 
Although the valley slopes are gentle, the ore-bed, which is readily permeable by 
water, has a great tendency to slide down over the underlying impervious Lias 
clays into the valleys. This large-scale sliding may introduce complications into 
observations taken along the edges of the sheet. 
Under the plateau, where the bed is covered up to a depth of 50 feet or more by 
impermeable clays of the Estuarine and Great Oolite series, the ore is mainly in the 
form of ferrous carbonate with some ferrous silicate. Along the surface outcrops the 
ore is completely oxidised to limonite. In intermediate positions the oxidation is 
still going on along the highly irregular jointing. With low barometric pressure 
such large volumes of COg are evolved from the rocks that workmen have to be with¬ 
drawn from the tunnels at present being driven through the deposit. 
There is a great difference in susceptibility between the oxidised and unoxidised 
portions of the ore, the latter giving, as would be expected, the higher values, as 
shown by the following figures [vide pp. 85 and 105) :— 
Volume- 
Iron content.* 
Susceptibility.* 
EeO. 
FesOs. 
N. 83 
Brown oxidised ore from the outcrop 
6 X 10-5 
Per cent. 
Per cent. 
77-28 
N. 82 
J) 
5> 
8 X 10-5 
-- 
44-76 
N. 89 
Green sandy carbonate . .' 
from 
22 X10-5 
40-68 
4-32 
N. 91 
„ carbonate .... 
28 X 10-5 
38-54 
0-31 
N. 88 
„ brown carbonate . . 
> underground 
32 X 10-5 
36-96 
12-00 
N. 90 
,, silicate.^ 
workings. 
36 X 10-5 
49-32 
0-80 
(E.W. and A.H.C.) 
Thus in general the outcrops of the sheet will be composed of less magnetic 
material than the more deeply buried portions. Even for these parts, however, the 
susceptibilities are low, less than half that of the Mount Sorrel granite. 
In view of the differences in the susceptibility of different parts of the sheet, the 
positions of the observation stations are important {see figs. 2 and 3). Two of the 
stations, A and B, are on the Great Oolite plateau well above the ore-bed. A is about 
in the centre of the plateau and almost exactly 100 feet above the ore, while B is near 
the eastern edge of the plateau and also about 100 feet above the ore. One station 
E is situated on the valley side and actually on the outcrop of the iron-ore. At this 
point, however, the usual slipping has taken place so that the thickness of the ore is 
The figures in each case refer to the air-dried specimen. 
VOL. CCXIX.-A. 
N 
