Vol. 50.] GNEISSES IN THE COTTIAN SEQUENCE. 205 



sufficient to show that an eruptive- or fluxion-foliation is well 

 established. 1 



Representatives of both the metainorphic aud the fluxion-gneisses 

 occur in the Cottians ; thus the gabbros (or zobtenites) of Monte 

 Viso are due to the deformation of massive gabbros, while the 

 angen-gabbro-gneiss along the margin of the gabbro intrusion at 

 Le Chenaillet, on Mont Genevre, 2 appears to be clearly due to 

 fluxion in a viscid rock near the contact. 



"We have then to consider which of the two explanations must 

 be applied to the Waldensian gneisses. The field evidence at once 

 suggests that the structure in these is a contact-fluxion. The fact 

 that the rock is foliated on the margin, while often granitic in the 

 centre of the massif, is not by itself conclusive. The stress of 

 subsequent earth-movements might readily set up a deformation 

 along the line of contact of two dissimilar rocks. But the evidence 

 of the dykes is free from such doubts ; the foliation is there parallel 

 to the walls, and is frequently at a high angle to that of the 

 neighbouring schists. The dykes often have an irregular course 

 through the schists, but the foliation in them remains quite in- 

 dependent of that of the surrounding rocks. This appeal's to show 

 conclusively that in these eases, at least, the foliation is a contact- 

 fluxion, and has no connexion with the dynamo-metamorphism of 

 the district. 



Microscopic examination shows the identity of the structures in 

 the apophyses and the massifs. In both the phenocrysts present 

 true erosion-structures, as well as some which — like those from the 

 Republic of Colombia discussed by Kuch 3 — may better be explained 

 as due to pressure-deformation. In both, the micas and other 

 constituents are distributed along lines of flow around the larger 

 crystals. Finally, there is no evidence of the production of 

 secondary minerals, such as invariably accompanies dynamo- 

 metamorphism sufficiently powerful to produce a molecular re- 

 arrangement of the constituents. 



(b) The Clastic Gneisses. — If gneiss be defined simply as a 

 foliated rock consisting of quartz, orthoclase, and mica, then there 

 are in the Eastern Cottians numerous rocks to which this name 

 must be applied : they were clastic rocks — now altered by the 

 intrusive Waldensian gneisses. These have been previously referred 

 to as gneissoid mica-schists, and one has already been described on 

 p. 259. That one serves as a convenient type, because all stages can 

 be traced to it from a lead-coloured schist of undoubtedly clastic 

 origin. 



1 Cases of foliation due to a kind of concretionary action on an enormous 

 scale are probably exceptional and local. Such, however, appears to be the 

 explanation of the foliation of some of the basic igneous rocks on the north- 

 west coast of Lake Superior, which the writer hopes shortly to describe. 



2 Cole and Gregory, ' The Variolitic Rocks of Mont Genevre,' Quart. Journ. 

 Geol. Soc. vol. xlvi. (1890) p. 303. 



3 Rich. Kuch, ' Petrograpuie— pt. i. : die vulkanischen Gesteine : ' in Reiss 

 & Si ubel's 'Geologische Studien in der Republik Colombia,' 1892, p. 61. 



Q. J. G. 8. No. 198. T 



