H. H. Read — Tlie Two Magmas of StratJibogie. 367 



during consolidation, and the effect of the subsequent cataclasis has- 

 been to emphasize the banded nature of the rock, which in 

 structure then resembles the foliated gabbro described and figured 

 by Teall. 1 All banding is parallel to the stress lines of the country. 



The olivine-pvroxene-felspar rock, enstatite-gabbro, diorite, and 

 banded gabbros do not form independent intrusions, but are small 

 parts of a heterogeneous intrusion whose predominant member is 

 ordinary gabbro. No differentiation in place can be demonstrated 

 and it is considered that this heterogeneity is due to differentiation 

 before intrusion. All the varieties of the gabbro pass into epidiorites 

 by the complete replacement of their pyroxene by secondary horn- 

 blende. From the epidiorites are derived the widespread amphi- 

 bolites by a still more abundant development of secondary hornblende 

 and the concomitant recrystallization of the original labradorite 

 into a mosaic of clear acid plagioclase. These two types, displaying 

 no marked foliation, are predominant in the Older basic series as 

 now found. By the development of cataclastic foliation the horn- 

 blende-schists are produced. 



The acid members of the Older Series are exemplified by the 

 augen-gneisses of Portsoy and Windyhills. In these rocks large 

 lens-shaped patches of microcline and orthoclase, with subordinate 

 oligoclase, are set in a cataclastic groundmass of granular quartz and 

 felspar with shreds of primary biotite, and much secondary muscovite 

 along shear planes. The original rock was a biotite microcline 

 granite. 



The magmatic sequence of the Older Series is, as usual, one of 

 decreasing basicity or of increasing alkalinity. The main magma 

 was largely gabbroic in nature and has been intruded as a hetero- 

 geneous magma due to differentiation in a lower basin, and now 

 displays the varied types of predominant gabbro, scarce olivine- 

 pyroxene-felspar rock, scarce enstatite-gabbro, scarce diorite, with 

 banded felspathic and pyroxenic portions. The manner of 

 differentiation cannot be stated, but no degree of differentiation 

 in situ has taken place. The anchi-monomineralic rocks are earlier 

 than the gabbro; one of these independent intrusions is composed 

 of the femic constituents, the other of the felspathic constituent of 

 the original gabbroic magma from which they have split. The acid 

 differentiate is quite late and consists of an alkali-granite. The 

 areal relations for this district of the three divisions are Tiltrabasic 

 rocks 26 square miles, basic 15 - 6 square miles, and acid Too square 

 miles. The basic rocks are much cut out by the Younger Series, 

 and the area formerly occupied by them may be approximately 

 double that given above. 



The Older Sei'ies must be considered with regard to Dr. N. L. 

 Eowen's 2 recent work on differentiation by the sorting and collection 

 of crystals in a crystallizing magma. At Portsoy among the Older 



1 J. J. H. Teall, Geol. Mag., 1886, PI. XIII.p. 481 ; British Petrography, 

 1888, pis. xxvi, xliii. 



2 N. L. Bowen, " The Last Stages in the Evolution of the Igneous Eocks " : 

 Journal of Geology, vol. xxiii, supplement, pp. 1-91, 1915. " The Problem 

 of the Anorthosites " : ibid., vol. xxv, p. 209, 1917. 



