106 DR. A. H. COX; REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
thoroughly acid rock in which iron-ores are relatively scarce, but consist entirely of 
magnetite, will have a high susceptibility. There is thus no direct connexion between 
the susceptibility and the basic or acid character of the rock. 
The amount of magnetite present in any particular igneous rock is controlled not 
merely by the amount of iron in the original magma, but also by the physico-chemical 
laws that govern crystallisation from complex solutions. One factor that evidently 
exerts a considerable influence is the form of combination assumed by the TiOa. In 
some rocks, the Mount Sorrel granite for example, part of the TiOa enters the biotite, 
the remainder combining with CaO to form sphene. In such a rock no TiOg is left 
free to combine with FeO to give ilmenite; consequently FeO is available to combine 
with FegOg giving magnetite, and the rock will have a high susceptibility. In other 
rocks, some of the dolerites, for example, TiOg enters the augite, and again FeO is 
available to form magnetite. But in other examples—the granophyres and certain 
dolerites—either owing to TiOg not going to the augite, or to its being present in 
excess, a certain amount is left free to combine with FeO giving ilmenite. Conse¬ 
quently there is little or no FeO available to give magnetite, and the rock will have 
a low susceptibility.' Adjustment between the two extremes depends upon a nice 
balance of conditions which may have varied within the limits of a single intrusion 
with consequent variations in the susceptibility values. The susceptibility of igneous 
rocks is, therefore, intimately connected with their petrography. Petrographical 
descriptions of the rocks tested are given in an addendum (p. 128). 
There is the further possibility that different magnetites have different suscepti¬ 
bilities, a subject that is being further investigated by Prof. Wilson. The relative 
size of the crystals would not appear to enter into the question, judging from the 
results of Prof. Wilson’s experiments on rock-powders (p. 87). Further experiments 
are in progress to ascertain whether the distribution of the magnetite exerts any 
controlling effect. The whole subject of the susceptibility of igneous rocks is in its 
infancy and much further work is needed. Meanwhile,'the following notes on the 
rocks tested may be of interest. 
(d) Rocks of Loiv Susceptibility. 
As already stated, rocks in which the accessory iron-ore is mainly or entirely 
ilmenite have low susceptibilities irrespective of whether they are acid or basic. The 
same is true for any rocks that are considerably weathered, or by reason of their age 
or position liave undergone changes analogous to weathering. Weathering results in 
the oxidation of any magnetite originally present, and consequently causes a rapid 
decrease in the susceptibility. 
The various rocks intrusive into the Charnian are among those that have low 
susceptibilities, due to their accessory iron-ore being in the form of ilmenite or its 
alteration-product leucoxene. This is true of the granophyres whether of acid or 
