8 14 THE POPULAR SCIENCE MONTHLY. 



ing 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 conse- 

 quently suggested that even volcanic phenomena may be con- 

 sequences 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 press- 

 ure 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 diame- 

 ter 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 uiD-thrust or subsidence. An 

 illustration may assist in forming a proper estimate of the irregu- 

 larities 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 16f miles, or 88,000 feet. On such a globe the difference be- 

 tween 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 dis- 

 tributed 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 



