Presidential Address. 209 



On the other hand, he would see that many of the mountain 

 ranges along the Pacific are comparatively new, and that 

 modern igneous action occurs in connection with them. 

 Thus he might be led to believe that the Atlantic, though 

 comparatively narrow, is an older feature of the earth's sur- 

 face ; while the Pacific belongs to more modern times. But 

 he would note, in connection with this, that the oldest rocks 

 of the great continental masses are mostly toward their 

 northern ends ; and that the borders of the northern ring of 

 land, and certain ridges extending southward from it, con- 

 stitute the most ancient and permanent elevations of the 

 earth's crust, though now greatly surpassed by mountains of 

 more recent age nearer the equator. 



Before leaving this general survey we may make one 

 further remark. An observer, looking at the earth from 

 without, would notice that the margins of the Atlantic and 

 the main lines of direction of its mountain chains are north- 

 east and south-west, and north-west and south-east, as if 

 some early causes had determined the occurrence of eleva- 

 tions along great circles of the earth's surface tangent to 

 the polar circles. 



We are invited by the preceding general glance at the sur- 

 face of the earth to ask certain questions respecting the 

 Atlantic. (1) What has at first determined its position and 

 form ? (2) What changes has it experienced in the lapse 

 of geological time? (3) What relations have these changes 

 borne to the development of life on the land and in the 

 water ? (4) What is its probable future ? 



Before attempting to answer these questions, which I 

 shall not take up formally in succession, but rather in con- 

 nection with each other, it is necessary to state, as briefly 

 as possible, certain general conclusions respecting the in- 

 terior of the earth. It is popularly supposed that we know 

 nothing of this beyond a superficial crust perhaps averag- 

 ing 50,000 to 100,000 feet in thickness. It is true we have 

 no means of exploration in the earth's interior, but the con- 

 joined labours of physicists and geologists have now pro- 

 ceeded sufficiently far to throw much inferential light on 

 the subject, and to enable us to make some general affirma- 



