and LEICESTERSHIEE and their relations to geological structure. 105 
comparable with that of the unoxidised carbonate ores from the Northampton Beds 
(see table below). 
It is noteworthy that the susceptibilities of the iron-ores so far examined (pp. 85 
and 86) appear to depend entirely on the chemical composition, and are not influenced 
by the texture of the deposits, and are to that extent independent of the geological 
horizon at which the ores occur. This is shown by the following figures :— 
Susceptibility. 
FeO. 
Fe203. 
Per cent. 
Per cent. 
f N. 83. 
Concretionary “ Boxstone ”. 
6 
— 
77-28 
N. 82. 
Red oolitic ore, Northampton Beds, near 
8-31 
— 
44-76 
Limonite 
- 
L. 71. 
Irthlingborough 
Red, fine-grained, vein-ore. Middle Lias, 
10 
53-12 
ores 
Leicestershire 
L. 63. 
Red oolitic ore, Middle Lias, Leicester- 
21 
trace 
58-88 
’N. 89. 
shire 
Green, sandy, oolitic ore, Northampton 
22 
40-68 
4-31 
Beds, Irthlingborough 
N. 91. 
Green oolitic ore, Northampton Beds, 
Irthlingborough 
28 
, 38-54 
0-31 
N. 88. 
Green-brown ore, Northampton Beds, 
29 
36-96 
12-00 
Carbonate 
N. 90. 
Irthlingborough 
Green-silicate ore, Northampton Beds, 
Irthlingborough 
36 
49 --32 
0-80 
L. 19. 
Clay-ironstone, Coal Measures .... 
34-5 
44-68 
0-26 
L. 28. 
21-5 
34-28 
2-32 
Chalybite (crystalline). 
59 
62-07 
— 
(theoretical) 
Ankerite, Coal Measures. 
10 
not esti 
mated 
(E.W. and A.H.C.) 
2. The Igneous Bocks. 
It has been found that different igneous rocks vary markedly as regards their 
volume susceptibility. This variation has no direct relation to the basic or acidic 
character of the rock. Acid rocks may have either high or low susceptibilities and 
similarly with basic rocks. The dominating factor that determines the susceptibility 
of an igneous rock is the amount of magnetite present in the rock, though there are 
doubtless other factors that play subordinate parts. Thus the presence of any 
mineral containing ferrous iron, e.g., magnetic pyrites, ilraenite, hornblende, &c., will 
tend to increase the susceptibility, but the effects of such minerals are feeble 
compared with that of magnetite itself. The accessory iron-ores in igneous rocks 
occur mainly in one or more of three forms, namely; (i.) magnetite ; (ii.) ilmenite ; 
(iii.) intergrowths of ilmenite and magnetite. Accordingly in those rocks in which 
the accessory iron-ore is present as ilmenite, FeTiOg, instead of magnetite, the 
susceptibility remains low even though the rock may be thoroughly basic with iron- 
ores occurring abundantly and in relatively large masses. On the other hand, a 
VOL. CGXIX.—A. 
p 
