Page:Popular Science Monthly Volume 33.djvu/834

814 matter; and it is argued, with apparent force, that no passage can exist by which molten matter, if there be any, could ascend from such depths to the surface. Recent speculation has consequently suggested that even volcanic phenomena may be consequences of the heat developed by intense pressures set up by the mechanical forces concerned in the movements of the cooling outer solid crust, and that they are not immediate results of the very high temperature which almost certainly still subsists at great depths in the earth's interior. A more probable explanation would seem to be that by some local or partial removal of pressure in the otherwise solid interior, a portion of intensely heated matter is able to pass into the fluid state, and so finds a way through some fissure to the surface.

Should any still hesitate to believe that vast mountains like the Himalaya or the Andes, and analogous depressions of the bed of the ocean, can have been produced by a mere secular change of the earth's temperature, I would remind them that the forces called into action by the earth are proportionate to its magnitude, and that their effects must be on a corresponding scale. It has been calculated on sound data that the contraction of the diameter of the earth, consequent on the fall of temperature from a fluid state to its present condition, has been about one hundred and ninety miles. At this rate a subsidence of five miles, which is the approximate greatest depth of the ocean, would correspond to a fall of temperature of about 200° Fahr. But the elevations and depressions of the earth's surface were probably produced by a comparatively much smaller loss of heat, and were due rather to tangential strains than to direct up-thrust or subsidence. An illustration may assist in forming a proper estimate of the irregularities of the earth's surface, which, though apparently great, are insignificant when viewed in relation to its actual dimensions. This hall might contain a globe forty feet in diameter. If this globe represented the earth it would be on a scale of one foot to about 200 miles; and one inch would be equivalent to a distance of 16 miles, or 88,000 feet. On such a globe the difference between the polar and equatorial diameters would be less than one inch, and the greatest elevations in Britain would be about the thickness of a threepenny-bit. The highest mountains and the deepest seas would be shown by elevations and depressions of hardly more than one third of an inch; and if they were distributed as such features are on the earth, they would be visible only with difficulty, and to the unaided eyes of a casual observer would hardly interfere with the apparent perfect smoothness of the globe's surface.

The conception of the vast duration of geological time is one with which most persons are now more or less familiar. It is