42 THE POPULAR SCIENCE MONTHLY. 



he would notice that those bounding the North Atlantic are in general 

 of great age, some belonging to the Laureutian system. 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 con- 

 nection 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 conti- 

 nental masses are mostly toward their northern ends, and that the bor- 

 ders of the northern ring of land and certain ridges extending south- 

 ward from it constitute 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 re- 

 mark. 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 northeast and southwest, and northwest and south- 

 cast, as if some early causes had determined the occurrence of elevations 

 along great circles of the earth's surface tangent to the polar circles. 

 We are invited by the preceding general glance at the surface 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 expe- 

 rienced in the lapse of geological time ? 3. What relations have these 

 changes borne to the development of life on the land and in the w T ater ? 

 4. What is its probable future? Before attempting to answer these 

 questions, which I shall not take up formally in succession, but rather 

 in connection with each other, it is necessary to state as briefly as pos- 

 sible certain general conclusions respecting the interior of the earth. 

 It is popularly supposed that we know nothing of this beyond a super- 

 ficial crust perhaps averaging fifty thousand to one hundred thousand 

 feet in thickness. It is true we have no means of exploration in the 

 earth's interior, but the conjoined labors of physicists and geologists 

 have now proceeded sufficiently far to throw much inferential light on 

 the subject, and to enable us to make some general affirmations with 

 certainty ; and these it is the more necessary to state distinctly, since 

 they are often treated as mere subjects of speculation and fruitless 

 discussion : 



1. Since the dawn of geological science, it has been evident that 

 the crust on which we live must be supported on a plastic or partially 

 Liquid mass of heated rock, approximately uniform in quality under 

 the whole of its area. This is a legitimate conclusion from the wide 

 distribution of y< Icanic phenomena, and from the fact that the ejec- 

 tions of volcanoes, while locally of various kinds, are similar in every 

 part of the world. It led to the old idea of a fluid interior of the earth, 

 but this is now gem rally abandoned, and this interior heated and plastic 

 layer is regarded as merely an under-crust. 



