Ixxiv PROCEEDINGS OF THE GHOLOGICAL socteTy. [May tgo2, 
as granite-margins are approached. In areas like the Lake District 
and the Vosges, where the contact-phenomena have been so well 
described by Ward and Rosenbusch, granite was intruded into 
folded sediments after the folding-stresses had ceased to act. There 
is no appreciable change in the chemical composition of the sedi- 
ments. The rocks are simply crystallized. 
But the contact-effects are not always so simple as in these cases. 
The intrusive origin of many encisses advocated long ago by Scrope 
and Darwin is now universally accepted. Any comparative study 
of contact-areas, therefore, must not be limited to granite-contacts, 
it must take into consideration also the phenomena associated with 
the intrusion of gneiss. Now, it is precisely by this comparative 
study of contact-areas that our ideas on the subject of metamorphism 
have been so greatly enlarged during the last decade of the century. 
The principle is one which has been applied with success in other 
branches of geology. It consists in studying a denudation-series. 
We cannot artificially prepare horizontal sections of the earth’s 
crust, so as to reveal the structure of a mass at different levels; but 
Nature has furnished us with a series of such sections in different 
areas, and by piecing these together in the proper order we can 
give an insight into the phenomena which belong to the deeper 
zones. The importance of this principle has been strongly emphasized 
by M. Michel Lévy, and applied by Prof. Barrois with most 
interesting results to the various granite-masses of Britanny. 
In connection with work of this kind a question of great 
importance has arisen: namely, the extent to which mixed rocks— 
or mictosites, as they may be called, if a special term be thought 
desirable—are produced. Although in many eases, especially those 
which belong to the higher levels, there is a sharp contrast between 
the intruding rock and that into which the intrusion has taken 
place, in others the contrast is less marked, and hand-specimens may 
be obtained which are neither igneous nor sedimentary, but a mixture 
of the two. I may illustrate the point by a case which came under 
my own observation—that of a cordierite-gneiss from Aberdeenshire. 
The igneous portion of the rock is composed of oligoclase, biotite, 
micropoikilitic orthoclase, and quartz; the sedimentary portion of 
cordierite, quartz, biotite, sillimanite, iron-ores, and a green spinel. 
The sedimentary rock into which the granite-magma was intruded 
is now represented by somewhat ill-defined shreds, patches, and 
streaks, in a paste of igneous origin. M. Michel Lévy recognizes 
two types of intermixture: the one taking place by superposition, 
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