MAGNETOTELLURIC MODELLING BY THE FINITE ELEMENT METHOD*

The finite element method, here viewed as a special case of the Galerkin projective method, is applied to the modelling of magnetotelluric problems, and its adaptation to geological profiles is outlined. A novel method for obtaining surface field values, involving matrix representation of the normal derivative operator, is presented in detail. Results obtained by this method are compared with well-known infinite series solutions for the vertical fault and the outcropping dyke. Two profiles containing sulphide zones are also modelled, the results being compared with field data; satisfactory agreement is obtained.     

The Crystallization of Intermediate and Acid Magmas at Ben Nevis, Scotland

The Ben Nevis igneous complex consists of a central mass ofrhyodacites surrounded by four plutonic intrusions. In orderof intrusion, these are: the Outer and Inner Quartz Diorites,which are complex rock types with varying chemical and mineralogicalcompositions; the Porphyritic Outer Granite; and the Inner Granite.The plutonic rocks crystallized under comparatively low pressureand thus contain no primary aluminou shornblende, its placebeing taken in all but the most acid rocks by primary pyroxene,which was commonly replaced by an alumina-poor hornblende atlower temperatures. The ferromagnesian minerals in the acidrocks are more magnesian than those in the more basic rocks.This is considered to be due to the partial pressure of oxygenremaining constant at an unusually high level as the magmasconsolidated, causing the minerals to be increasingly enrichedin magnesium as crystallization proceeded.