AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 107 
basic types, and for the porphyroids or altered andesitic rocks, the figures of the 
three types being as follows :— 
Acid granophyre or “ syenite,” Groby Quarry.4’0 
Basic „ “ greenstone,” Newhurst Wood . . 8'2 
Porphyroid (or altered dacite). High Sharpley.1 '3 
(E.W.) 
The last-named rock has the lowest susceptibility of any of the igneous rocks 
tested. 
Another rock with a relatively low susceptibility is the “ camptonite ” from the 
Cambrian of Nuneaton, the value found being 21. Accessory iron-ores were 
abundant in this rock, but they appeared to consist largely of ilmenite. The rock 
was, however, considerably weathered, and this maj" have further contributed in 
causing a low susceptibility. 
The low value 12'5 for the basalt lava (toadstone) from the Carboniferous Limestone 
of Derbyshire is probably the result of weathering. 
(e) The Mount Sorrel (dranite and. Associated Rocks. 
The susceptibility values for the various rock types occurring in or associated with 
the Mount Sorrel mass vary considerably, as seen from the following figures 
L. 
15. 
Normal granite—grey variety. 
65-130 
L. 
15a. 
,, ,, ,, adjacent to “ heathen” 
L. 015a .... 
135 
L. 
16a. 
,, „ pink. 
45 
L. 
100. 
Basic marginal granite. 
22 
L. 
015a. 
Basic patch in grey granite (“ heathen ”).... 
181 
L. 
016a. 
„ „ pink „ ,, .... 
40 
L. 
101. 
Hornblende diorite. 
12 
L. 
103. 
Basic dyke cutting granite. 
80 
(E.W.) 
Thus the susceptibilities of the various basic rocks which are associated with the 
Mount Sorrel granite are not such that any magnetic effects due to the normal 
granite would be materially increased by the presence of the basic rocks. In fact, 
the basic marginal granite and the hornblende-diorite have actually lower suscepti¬ 
bilities than the typical granite. 
In all these granitic rocks the accessory iron-ores are entirely in the form of 
magnetite, with the possible exception of the basic marginal granite in which some 
of the primary iron-ores may be sulphides. The presence of this magnetite readily 
accounts for the high susceptibilities of some of the rocks. But the amount of 
p 2 
