1892, 



THE SEQUENCE OF PLUTONIC ROCKS. 299 



peridotites, and the newest are eurites, composed almost entirely of 

 quartz and orthoclase. Between these two extremes occur augite- 

 diorites, diorites, quartz-diorites, and biotite-granites. The silica 

 ranges from 38-6 per cent, to 75-8 per cent. It seems impossible to 

 avoid the conclusion that this complex owes its origin to the differen- 

 tiation of an originally homogeneous magma and that the cause of the 

 differentiation is to be sought for in the order in which the minerals 

 successively form in igneous magmas. 



So far we have been mainly concerned with large areas of 

 plutonic rock ; but intrusive masses of no great size often show a 

 want of uniformity in composition such as can only be accounted for 

 by differentiation before or during consolidation. As illustrations we 

 may mention the felsitic and granophyric patches and veins in the 

 Carboniferous dolerites of Scotland, as, for example, those in the 

 neighbourhood of Stirling, and the corresponding phenomena in 

 many mica-traps. A rude kind of cellular structure may often be 

 observed in the latter rocks, the cell-contents being richer in ferro- 

 magnesian constituents than the cell-walls. 



We see, therefore, in this, as in so many other branches of geology, 

 that illustrations of the principles which enable us to explain the 

 structure of large areas may be observed in hand specimens or even 

 microscopic slides. If the differentiation theory be correct, the 

 micro-pegmatitic interstitial matter in an augite-diorite has originated 

 in the same way as the mountain-making granophyres of the Inner 

 Hebrides. The facts to which reference has been made appear to 

 show that a more or less gradual sequence from basic to acid rocks is 

 of very frequent occurrence, and that such a sequence can be corre- 

 lated with the order in which minerals are known to form in igneous 

 magmas. 



REFERENCES. 



1. Brogger, W. C. — Die Mineralien der Syenitpegmatitgange der siidnor- 



wegischen Augit und Nephelinsyenite. Zeitschvift f't'tr Krystallographie und 

 Mineralogie, vol. xvi., 1890. 



2. Iddings, J. P.— The Mineral Composition and Geological Occurrence of 



certain Igneous Rocks in the Yellowstone National Park. Bull. Phil. Soc, 

 Washington, vol. xi., pp. 191-220, 1890. 



3. Lainrson, Andrew C. — Petrographical Differentiation of certain Dykes of the 



Rainy Lake Region. Americaji Geologist, March, 1891. 



4. Vogt, J. H. L. — Om dannelsen af de vigtigste i Norge og Sverige repre- 



senterede grupper af jerumalmforekomster. Geologiska Fuieningeiis i Stockholm 

 Forhandlingar, vol. xiii., p. 476, iSgi. 



5. Dakyns and Teall.— On the Plutonic Rocks of Garabal Hill and Meall 



Breac. Quart. Joiivn. Geol. Soc, vol. xlviii., p. 104, 1892. 



6. Rosenbusch, H. — Ueber die chemischen Beziehungen der Eruptivgesteine. 



Mineralog. und Petiog. Mitth., vol. xi., p. 144, 1S89. 



J. J. H. Teall. 



