314 J. A. THOMSON. 



mental chemical peculiarities to allow it to give rise to a 

 secondary magma capable of producing these minerals, 

 instead of common augite. It is this chemically peculiar 

 primary magma which is necessary for the establishment 

 of a petrographical province. 



A further line of evidence which strengthens the writer's 

 suggestion is the recurrence of quartz-dolerites at different 

 geological ages in some of the above fragments of Gondwana 

 Land. Owing to their degree of alteration, it is not possible 

 to assert that the older groups also contained enstatite- 

 augite, but analyses support the view. This phase of the 

 subject is too extended to discuss at length here, but the 

 following facts may be instanced. Among the Western 

 Australian amphibolites of supposed pre-cambrian age there 

 are rocks which can be shown to be merely uralitised and 

 saussuritised quartz-dolerites. 1 Henderson 2 has described 

 similar rocks from the Transvaal, which may be assumed 

 to be much older than the Karroo dolerites. 



In India, the important fragment of Gondwana Land from 

 which Mesozoic quartz-dolerites have not been noted, there 

 is a well known occurrence of the rock in the Cuddepah, 

 and in these Wahl has shown the presence of enstatite- 

 augite. But there is also in the Archaean of India the 

 peculiar group of charnockites which exhibit, not indeed 

 the same structural peculiarities but very similar chemical 

 relations, viz. a high proportion of magnesia and iron com- 

 pared to non-felspathisable lime combined with an excess 

 of silica. 4 



1 Thomson, J. A., Petrographical Notes to Bull. 33, GeoL Surv. W.A., 

 1909, pp. 137, 145, 151, and 156. 



2 Henderson, J. A. L., Petrograpical and Geological Investigations on 

 certain Transvaal Norites, Gabbros and Pyroxenites, London, 1898, pp. 

 29 - 33. 



3 Holland, T. H., On Augite Diorites with Micropegmatite in S. India. 

 Q.J.G.S., liii, (1897) p. 405. Wahl, loc. cit. 



* Holland, T. H., The Charnockite Series, a group of Archaean Hypers- 

 thene Rocks in Peninsular India. Mem. Geol. Surv. India, xxvin.,Pt. 2, 

 pp. 119-249, 1900. 



