12 REPORT— 1886. 



It will be observed, in regard to these theories, that none of them 



Bnpposes that the old gneiss is an ordinary sediment, but that all regard 



it as formed in excepLlonal circumstances, these circumstances being the 



absence of land and of sub-aerial decay of rock> and the presence wholly or 



principally of the material of the upper surface of the recently hardened 



crust. This being granted, the question arises. Ought we not to combine 



these several theories and to believe that the cooling crust has hardened 



in successive layers from without inward ; that at the same time fissures 



were locally discharging igneous matter to the surface ; that matter held 



in suspension in the ocean and matter held in solution by heated waters 



rising from beneath the outer crust were mingling their materials in the 



deposits of the primitive ocean ? It would seem that the combination of 



all these agencies may safely be invoked ag causes of the pre-Atlantio 



deposits. This is the eclectic position which I endeavoured to maintain 



in my address before the Minneapolis Meeting of the American Association 



in 1883, and which I still hold to be in every way probable. 



A word here as to metamorphism, a theory which, like many others, has 

 been first ruu to death and then discredited, but which to the moderate de- 

 gree in which it was originally held by Lyell is still valid. Nothing can be 

 more certain than that the composition of the Laurentian gneisses forbids 

 us to suppose that they can be ordinary sediments metamorphosed. They 

 are rocks peculiar in their origin, and not parallelled unless exceptionally 

 in later times. On the other hand, they have undoubtedly experienced 

 very imnortant changes, more especially as to crystallisation, the state of 

 combii .ition of their ingredients, and the development of disseminated 

 minerals ; ^ and while this may in part be attributed to the mechanical 

 pressure to which they have been subjected, it requires also the action of 

 hydrothermic agencies. Any theory which fails to invoke both of these 

 kinds of force must necessarily be partial and imperfect. 



by the central mass, was the source of mineral springs, holding in solution the silicates 

 which built up the ancient gn 3isses and similar rocks. Granitic veins and zeolites 

 are due to survivals of the pre jess which generated the gneissic rocks. The hypo- 

 thesis of their formation from materials brought to the surface by mineral springs 

 foom the primitive basic layrr affords, it is claimed, the elements of a complete and 

 intelligible explanation of the origin of the Eozoic rocks. This upward lixiviation of 

 the primitive mass, and the c eposition over it of an acidic granite- like rock, would 

 leave below a highly basic material, and the division of the mass thus established 

 would correspond to that of th<3 trachytic and doleritic magmas, which have been 

 conjectured to be the sources of two great tj-^pes of eruptive rocks. Inasmuch, how- 

 ever, as according to the present hypothesis these two layers of basic and acidic 

 matters are the results of aqueous action, and not of an original separation in a 

 plutonic mass, as imagined by Phillips and Durocher, their composition would be 

 subject to many local variations.' 



• The first of these is what Bonney has called Metastasis. The second and third 

 come under the name Metacrasis. Methylosis, or change of substance, is altogether 

 exceptional, and not to be credited except on the best evidence, or in cases where 

 volatile matters have been expelled, as in the change of Hematite into Magnetite, or 

 of bituminous coal into anthracite. 



