TRANSACTIONS OF SECTION C. 741 



answering to sandstones, shales, and limestones. Apply this test to the gneisses of 

 Scotland, and I helieve it will be found in many cases to furnish a solution of the 

 problem. Caution, however, is necessary ; for crystal-building and the formation of 

 segregation veins and patches in the sedimentary schists clearly prove that a 

 migration of constituents takes place under certain circumstances. 



Recent work on tlie gneisses and schists of igneous composition has shown that 

 the parallel structure, by no means invariably present, is sometimes the result of 

 fluxion during the final stages of consolidation, and sometimes due to the plastic 

 deformation of solid rocks. When compared with masses of ordinaiy plutonic 

 rock, the principal points of diftljrence, apart from those due to secondary dynamic 

 causes, depend on what may be called their extreme petrographical differentiation. 

 Indications of differentiation may, however, be seen in the contemporaneous veins 

 and basic patchee so common in ordinary irruptive bosses, but they are never so 

 marked as in gneissic regions, like those of the north-west of Scotland, where 

 specimens answering in composition to granites, diorites, and even peridotites 

 may be collected repeatedly in very limited areas. The nearest approach to the 

 conditions of gneissose regions is to be found in connected masses of diverse 

 plutonic rocks, such as those which are sometimes found on the borders of great 

 granitic intrusions. 



The tectonic relations of those gneisses which resemble igneous rocks in 

 composition fully bear out the plutonic theory as to their origin. Thus, the intru- 

 sive character of granitic gneiss in a portion of the Himalayas has been demon- 

 strated by General McMahon.^ The protogiue of Mont Blanc has been investigated 

 by M. Levy - with the same result. Most significant of all are the discoveries in 

 the vast Archiean region of Canada. Professor Lawson ^ has shown that immense 

 areas of the so-called Laurentian gneiss in the district north-west of Lake Superior 

 are intrusive in the surrounding rocks, and therefore newer, not older, than these. 

 Professor Adams* has quite recently established a similar fact as regards the 

 anorthosite rocks— the so-called Norian — of the Saguenay River and other districts 

 lying near the eastern margin of the ' Canadian shield.' Now that the intrusive 

 character of so many gneisses is being recognised, one wonders where the tide of 

 discovery will stop. How long will it be before the existence of gneisses of Tertiary 

 age will be generally admitted ? At any rate, the discoveries of recent years have 

 compelled the followers of Wernerian methods to evacuate large slices of territory. 



Turning now to the gneisses and schists which resemble sedimentary rocks in 

 composition, we note that the parallel structure may be due to original stratifica- 

 tion, to subsequent deformation, or to both of these agencies combined. It must 

 also be remembered that they have often been injected with igneous material, as 

 Hutton pointed out. Where this has followed parallel planes of weakness, .we 

 have a banding due to alternations of igneous and sedimentary material. This 

 injection lit par lit has been shown by M. Levy to be a potent cause in the forma- 

 tion of certain banded gneisses. 



Will the various agencies to which reference has been made explain all the 

 phenomena of the crystalline schists and gneisses ? I do not think that the present 

 state of our knowledge justifies us in answering this question in the affirmative. 

 Those who are working on these rocks frequently have brought under their notice 

 specimens about the origin of which they are not able to speak with any degree of 

 confidence. Sometimes a flood of light is suddenly thrown on a group of doubtful 

 rocks by the recognition of a character which gives unmistakable indications of 

 their mode of origin. Thus, some of the fine-grained quartz-felspathic rocks asso- 

 ciated with the crystalline schists of the Central Highlands are proved to have 



' ' The Geology of Dalhousie,' Records of Geol. Survey of India, vol. xv. part 1 

 (1882), p. 34. See also vol. svi. part 3 (1883), p. 12t). 



- ' Les Roches Crystallines et Eruptives des Environs du Mont-Blanc,' Bull, deg 

 Services de la Carte Geologique de la France, No. 9 (1890). 



3 ' On the Geology of the Rainy Lake Region,' Annual Rejiort Geol. Survey of 

 Canada for 1887. 



* ' Ueber das Norian oder Ober-Laurentian von Canada,' Neuet Jahrluch f. Mine' 

 ralogie, &c., Beilage, Band viii. p. 419. 



