in the Cortland t Series. 129 



of shearing ; and that in all the cases examined which show 

 this flow structure, it is ahsent, even in thin section. The phe- 

 nomenon could not, therefore, be due to ordinary regional 

 metamorphism. 



The classic locality for this structure is on the Isle of Skye, 

 in Tertiary gabbro, where it was first described as such by Sir 

 Archibald Geikie and Professor Teall.* From their photo- 

 graphs it appears to be little, if any, better developed than 

 in the Cortlandt. The same structure, however, has been 

 described in wonderful perfection by Professor A. G. Hogbom* 

 from the Island of Orno, just south of Stockholm. Here the 

 black and white bands are narrow and numerous, and may be 

 traced for 60-80 meters with the utmost ease. The zone in 

 which this development occurs constitutes the periphery of an 

 igneous (dioritic) complex. It is also found strikingly devel- 

 oped near Montreal; this occurrence will be described by 

 Professor Frank D. Adams in a forthcoming work. 



The explanation now accorded this phenomenon is simple 

 and plausible. Eliminating Dana's idea of workecl-over vol- 

 canic ashes, and Williams' suggestion of the ordinary regional 

 metamorphism of igneous rocks, we are thrown back on some 

 force concomitant in its action with the cooling of the rock. 

 Since the several layers or streaks are always quite different 

 in mineralogical composition, and more or less so in chemical, 

 it is evidently a question of magmatic differentiation. It is 

 inconceivable that the structure be due to the differentiation 

 of a magma in situ — after it has reached its present position 

 — since the differentiation is into bands which bear no definite 

 relation to the borders of the magma; and the idea of success- 

 ive intrusions— first of a light band and then of a dark — is equally 

 inapplicable, since even when there is a sharp line of demar- 

 cation separating two bands, the individual grains seem to 

 interlock across the line. The only remaining hypothesis, 

 therefore, is that of the intrusion of a molten mass already 

 heterogeneous. Professors Geikie and Teall ^ push their con- 

 clusions thus far ; Mr. Ilarker § goes a bit further. He appears 

 to favor the view that the structure is due to the approxi- 

 mately simultaneous intrusion of two different magmas, which 

 would give rise to a thorough interpenetration of the two. 

 This would explain why the banded structure, which always 

 shows evidence of flowage, is seldom straight and clear-cut. 

 The assumption would be then that the mass must have 



* On the Banded Structure of some Tertiary Gabbro on the Isle of Skye. 

 Quar. Journ. Geol. Soc, Nov. 1894, i, 046. 



fZur Petrographie von Orno Hufvud, Bull. Geol. Instit. TJpsala, x, 150. 



% Op. cit. 



§ Natural History of Igneous Kocks, New York, 1909. 



