PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 223 



the notion that it is due to mere distortion of schlieren structures by 

 movement akin to flow, is. a much simpler explanation and one which 

 has many parallels amongst rocks whose characters are better 

 understood. 



That banding of a kind indistinguishable from that exhibited by 

 the gneisses occurs in undoubted eruptives is shown by the striking 

 instances occurring in the tertiary basic igneous rocks of West Scot- 

 land. On account of the resemblance of these banded rocks to some 

 of the old gneisses and pyroxene-granulites of the adjoining areas, 

 they were at one time considered to be portions of the old 

 Archaean complex ; but Sir A. Geikie and Mr. Teall have shown that 

 the rocks are merely local modifications of the well known tertiary 

 gabbros. 1 After considering two explanations, namely, differentia- 

 tion in situ and successive intrusions, Geikie and Teall concluded 

 that the banded structure of these gabbros is the result of the intru- 

 sion of a heterogeneous, that is, as Reyer would say, of a " schlierig," 

 magma. 



That banding may be produced, however, in some cases by 



successive injection is shown by the gneisses 

 Lit-par-lit injection. . " ^ . 



in the neighbourhood of I irrupur, Loimbatore 

 District. But in these rocks the banding is of a much more 

 definite nature than that usually exhibited by the charnockite series. 

 Although they present the essential features of the charnockite series, 

 the rocks in the neighbourhood of Tirrupur contain an unusual 

 amount of hornblende, and the non-felspathic forms might almost 

 be described as hornblende-rocks. The latter occur as numerous, 

 narrow, well-defined bands, separating the basic or intermediate 

 felspathic forms, and giving the whole rock an extremely distinct 

 banded structure, with a constant W.-N.-W. — E.-S.-E. strike. 

 When the black hornblende bands, however, are carefully followed, 

 they are found to run with the general foliation for some distance, 

 then suddenly break across the folia and again continue their 

 original direction, though in a different line. In a few cases the 

 1 Quart. Journ. Geol, Soc, Vol. L (1894), pp. 645-659. 



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