AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 109 
The intergrowth may show the external habit of magnetite or of ilmenite, and this 
may lead to apparent anomalies in the susceptibility results. Thus an estimation of 
the amount of iron-ore present, even in rocks in which the iron-ore appears to be all 
magnetite, does not necessarily yield a measure of the susceptibility, though it may 
be a guide in many, or even in the majority, of cases. 
The susceptibility of weathered dolerites or basalts is low, as in all weathered 
rocks, even if the iron-ore content was originally high, as in the altered upper portion 
of the Southwell rock. . 
3. Summary. 
The study of the magnetic susceptibilities of igneous rocks is in its infancy; the 
susceptibility is controlled by a number of factors the precise effects of which are at 
present unknown. It evidently depends mainly upon the amount of magnetite 
present, and is therefore independent of the acid or basic character of the rock. 
The Mount Sorrel granite Serves as an example of a rock with magnetite as the 
principal iron-ore, with resultant high susceptibility. The Charnwood (Groby) 
granophyre is similar to the granite as regards degree of acidity and minerals present, 
but its iron-ore is entirely in the form of ilmenite, hence the low susceptibility value. 
In dolerites the minerals ilmenite and magnetite are frequently intergrown, some¬ 
times the one, sometimes the other predominating. This may result in rapid varia¬ 
tions of the susceptibility within the limits of a single intrusion. As a general rule, 
however, dolerites have high susceptibilities which are specially marked in dolerites of 
the type intrusive into Coal Measures. 
III. Correlation of the Magnetic Results with the Composition and 
Tectonics of the Rocks. 
Mr. Walker has produced evidence (Part I., p. 82) for the existence of “ mag¬ 
netic peaks” at three localities : (i.) Nottingham; (ii.) near Rempstone ; (iii.) Melton 
Mowbray, with perhaps a fourth smaller “ peak ” near Pen Hill (see map, Plate Ap. 3). 
The various rocks which in view of their iron content might conceivably cause 
magnetic disturbances, and which are known to occur in the district, are:— 
(I.) The Northampton Iron-ores of the Inferior Oolite. 
(II.) The Marlstone ,, ,, Middle Lias. 
(HI.) Dolerites intrusive into the Coal Measures. 
(IV.) The Clay-ironstone of „ ,, 
(V.) Granites intrusive into pre-Carboniferous Rocks. 
(VI.) Granophyres intrusive into Charnian Rocks. 
(VH.) Andesitic intrusions and lavas in Charnian Rocks. 
