Presidential Address. 215 



If we now inquire as to the cause of the Atlantic depres- 

 sion, we must go back to the time when the areas occupied 

 by the Atlantic and its bounding coasts were parts of the ' 

 shoreless sea in which the earliest gneisses or stratified 

 granites of the Laurentian age were being laid down in vastly 

 extended beds. These ancient crystalline rocks have been 

 the subject of much discussion and controversy, and as they 

 constitute the lowest and probably the firmest part of the 

 Atlantic sea-bed, it is necessary to inquire as to their origin 

 and history. Dr. Bonney, the late President of the Geolo- 

 gical Society, in his Anniversary address, and Dr. Sterry 

 Hunt, in an elaborate paper communicated to the Eoyal 

 Society of Canada, have ably summed up the hypotheses as 

 to the origin of the oldest Laurentian beds. At the basis of 

 these hypotheses lies the admission that the immensely thick 

 beds of orthoclase gneiss, which are the oldest stratified 

 rocks known to us, are substantially the same in composition 

 with the upper or silicious magma or layer of the under- 

 crust. They are, in short, its materials either in their primi- 

 tive condition or merely re-arranged. One theory considers 

 them as original products of cooling, owing their lamination 

 merely to the successive stages of the process. Another 

 view refers them to the waste and re-arrangement of the 

 materials of a previously massive granite. Still another 

 holds that all our granites really arise from the fusion of old 

 gneisses of originally aqueous origin, while a fourth refers 

 the gneisses themselves to molecular changes effected in 

 granite by pressure. These several views, in so far as they 

 relate to the oldest or fundamental Laurentian gneiss, may 

 be arranged under the following heads : (1) Endoplutonic, or 

 that which regards all the old gneisses as molten rocks cooled 

 from without inward, in successive layers. 1 (2) Exoplutonic, 

 or that which considers them as made up of matter ejected 

 from below the upper crust in the manner of volcanic action 2 . 

 (3) Metamorphic, which supposes the old gneisses to arise 

 from the crystallisation of detrital matter spread over the 

 sea-bottom, and either igneous or derived from the decay of 



1 jSTaumann, Phillips, Durocher, Macfarlane, &c. 



2 Clarence King, Tornebohm, Marr. &c. 



