Presidential Address. 21*7 



later times. On the other hand, they have undoubtedly 

 experienced very important changes, more especially as to 

 crystallization, the state of combination of their ingredients, 

 and the development of disseminated minerals ; l 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. 



But all metamorphic rocks are not of the same character 

 with the gneisses of the Lower Laurentian. Even in the 

 Middle and Upper Laurentian, we have metamorphic rocks, 

 e. g. quartzite and limestone, which must originally have 

 been ordinary aqueous deposits. Still more, in the succeed- 

 ing Huronian and its associated series of beds, and in the 

 Lower Palaeozoic, local metamorphic change has been under- 

 gone by rocks quite similar to those which in their unaltered 

 state constitute regular sedimentary deposits. In the case 

 of these later rocks it is to be borne in mind that, while some 

 may have been of volcanic origin, others may have been 

 sediments rich in undecomposed fragments of silicates. It 

 is a mistake to suppose that the ordinary decay of stratified 

 siliceous rocks is a process of kaolrnization so perfect as to 

 eliminate all alkaline matters. On the contrary, the fact, 

 which Judd has recently well illustrated in the case of the 

 mud of the Nile, applies to a great number of similar de- 

 posits in all parts of the world, and shows that the finest 

 sediments have not always been so completely lixiviated as 

 to be destitute of the basic matters necessary for their con- 

 version into gneiss, mica-schist, and similar rocks, when the 

 neceesary agencies of metamorphism are applied to them, 

 and this quite independently of any other extraneous matters 

 introduced into them by water or otherwise. Still it must 



1 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 haematite into magnetite, 

 or of bituminous coal into anthracite. 



15 



