Vol. 50.] ANNIVERSARY ADDRESS OF THE PRESIDENT. 1 1 7 



parallel, since in the Norwegian case the rocks are all more or less 

 basic, nor do we know the amount of field-evidence on which the 

 genetic connexion between these several bosses is established. 

 With regard to the rocks on the west side of Glenfalloch, we 

 seem to have an assurance that such a sequence as the Authors have 

 narrated actually exists within the area in question, and conse- 

 quently that the important deductions which follow do not wholly 

 depend upon petrological considerations. It is all very well to 

 determine the order of crystallization of minerals in the cabinet, 

 but if this philosophy is to be applied to large areas the evidence in 

 the field must be above suspicion. 



Very much the same views, as to the effects resulting from the 

 order of crystallization, are expressed by Mr. Uarrow in his paper 

 on an Intrusion of Muscovite-biotite Gneiss in the South-eastern 

 Highlands. The normal condition of the intrusive rock is that of a 

 slightly foliated granite with two micas. The masses observed 

 vary in size, and the larger ones are more or less fringed with 

 pegmatite - veins, which cut the metamorphic schists in every 

 direction, the foliation in the larger masses being rudely parallel to 

 that of the surrounding schists. In the north-western portion of 

 the area the intrusive rock is always a gneiss, and occurs in thin 

 tongues which permeate the surrounding rocks. Towards the south- 

 east these tongues amalgamate, and form large masses in which the 

 foliation is less marked. Where the rock is a gneiss, it is composed 

 of oligoclase, muscovite, biotite, and quartz, but contains no 

 microcline. As the gneissic character becomes less marked, the 

 oligoclase diminishes in amount, and microcline begins to appear, 

 especially towards the margin of the masses, and in the most 

 south-easterly of these microcline is greatly in excess of oligoclase. 

 The differences in structure and composition are believed by the 

 Author to be due to the straining off of the crystals of earlier 

 consolidation during intrusion under great pressure. The still 

 liquid potash-bearing portion of the magma was squeezed out and 

 forced into every plane of weakness in the surrounding rocks ; 

 and that portion of it which contained the highest percentage of 

 potash finally consolidated as pegmatite. 



The phenomena of thermo-metamorphism, accompanying this 

 intrusion of plutonic rock, are next considered. The rocks into 

 which these igneous masses have been intruded are in a highly 

 crystalline condition, and their grain is coarse, the micas being 

 especially of large size. The general aspect of the north-western 



