GEOLOGY OF THE ATLANTIC OCEAN. 45 



much a depression of the crust of the earth as a flattening of it ; and 

 this, as recent soundings have shown, with a slight ridge or eleva- 

 tion along its middle, and hanks or terraces fringing the edges, so that 

 its form is not so much that of a basin as that of a shallow plate with 

 its middle a little raised. Its true, permanent margins are composed 

 of portions of the over-crust folded, ridged up, and crushed as if by 

 lateral pressure emanating from the sea itself. We can not, for ex- 

 ample, look at a geological map of America without perceiving that 

 the Appalachian ridges, which intervene between the Atlantic and the 

 St. Lawrence Valley, have been driven bodily back by a force acting 

 from the east, and that they have resisted this pressure only where, as 

 in the Gulf of St. Lawrence and the Catskill region of New York, 

 they have been protected by outlying masses of very old rocks, as, for 

 example, by that of the Island of Newfoundland, and that of the Adi- 

 rondack Mountains. The admirable work begun by my friend and 

 fellow-student Professor James Nicol, followed up by Hicks, Lap- 

 worth, and others, and now, after long controversy, fully confirmed 

 by the recent observations of the geological survey of Scotland, has 

 shown the most intense action of the same kind on the east side of 

 the ocean in the Scottish Highlands ; and the more widely distributed 

 Eozoic rocks of Scandinavia may be appealed to in further evidence 

 of this. 



If we now inquire as to the cause of the Atlantic depression, we 

 must go back to a time when the areas occupied by the Atlantic and 

 its bounding coasts were parts of- a 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 At- 

 lantic sea-bed, it is necessary to inquire as to their origin and history. 

 Dr. Bonney, the late President of the Geological Society, in his anni- 

 versary address, and Dr. Sterry Hunt, in an elaborate paper com- 

 municated to the Royal 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 

 siliceous magma or layer of the under-crust. They are, in short, its 

 materials either in their primitive condition or merely rearranged. 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 rearrangement of the materials of 

 a previously massive granite. Still another holds that all our gran- 

 ites 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. 



