264 DR. J. W. GREGORY ON THE WALDENSIAN [May 1 894, 



of some of the intrusions, the contact-alterations are comparatively 

 slight, and we have no such striking changes as those to be seen 

 around the Biella porphyries, for example. This therefore suggests 

 that the rock when intruded was not at a very high temperature, 

 but in the viscid condition of a mass that has undergone partial 

 consolidation. This is in full agreement with the evidence of the 

 fluxion-structures. It is probable that most if not all of the true 

 fluxion-gneisses were formed under similar conditions, and we cannot 

 therefore expect contact-metamorphism so extensive as that produced 

 by the intrusion of rocks, all the constituents of which were molten 

 at the time of injection. 



VI. The Origin of the Gneissic Structure. 



(a) The Igneous Gneisses. — Before DarM 7 in's * discussion of the 

 subject in 1846, the view of the origin of the foliated rocks by 

 original sedimentation had been universally held. Though the 

 objections Darwin advanced have now been generally recognized as 

 insuperable, the theory long flourished and lingers yet. In its 

 stead have been substituted the agencies of d3namo-metamorphism, 

 fluxion in semi-viscid consolidating rocks, and a more local combined 

 fluxion and concretionary action. The first of these is that which 

 is most widely applicable and is the real cause of the parallel folia- 

 tion of rocks of very different composition over wide areas, i. e. of 

 all cases of regional metamorphism. But that this is not a universal 

 explanation is now admitted. Thus it has long been known that 

 many intrusive rocks which are normally granitic in structure 

 become foliated around the margins and in the apophyses which run 

 oft" from them. Prof. Bosenbusch, 2 for example, clearly recognizes 

 gneisses thus formed as well as those due to dynamo-metamorphic 

 action. So also do MM. Michel-Levy and Barrois. 3 In England 

 the same theory has been extended to cases previously regarded as 

 due to dynamo-metamorphic action. Thus it has been applied by 

 Prof. Bonney and General M c Mahon 4 to the foliated gabbros of 

 Cornwall, by the latter author 6 to certain gneis^ose granites of the 

 Himalaya, by Prof. Bonney and the Bev. Edwin Hill 6 to the banded 

 gneisses of Sark, and by Sir Archibald Geikie 7 to many Archaean 

 gneisses. Further illustrations could easily be added, but this is 



1 Darwin, ' Geological Observations on South America,' 1846, chap. vi. 



2 Rosenbusch, 'Mikrosk. Physiogr. Mass.Gesteine,' 2nd ed. vol.ii. (1887) p. 41. 



3 Barrois, ' Le Granite de Rostrenen, ses Apophyses et ses Contacts,' Ann. 

 80c. geol. Nord, vol. xii. (1885) p. 105. 



4 ' On the Crystalline Rocks of the Lizard District,' Quart. Journ. Geol. Soc. 

 vol. xlvii. (1891) pp. 483-491. (Further references given in footnote, p. 489.) 



5 'Note on the Foliation of the Lizard Gabbro,' Geol. Mag. - 1887, pp. 74-77. 

 e • On the Hornblende-Schists, Gneisses, and other Crystalline Rocks of Sark,' 



Quart, Journ. Geol. Soc. vol. xlviii. (1892) pp. 125-127, 132-137. Note by 

 Prof. Bonney, ibid. pp. 145, 146. 



7 In discussion of Prof. Bonney and General M c Mahon's paper, op. tit. 

 vol. xlvii. (1891) p. 499. 



