MOUNTAIN CHAINS 31 



estimate of the age of the oceans. We may therefore 

 turn to a second alternative. As regards this it is by no 

 means certain that the exposed portion of the Penokee 

 series ever was depressed 50,000 feet : the beds lie in a 

 synclinal the base of which, indeed, may have sunk to this 

 extent, and entered a region of metamorphosis ; but the 

 only part of the system that lies exposed to view is the 

 upturned margin of the synclinal, and as to this it would 

 seem impossible to make any positive assertion as to the 

 depth to which it may or may not have been depressed. 

 To keep an open mind on the question seems our only 

 course for the present, but difficulties like these offer a 

 promising field for investigation. 



It is frequently alleged that mountain chains cannot be 

 explained on the hypothesis of a solid earth cooling under 

 the conditions and for the period we have supposed. This 

 is a question well worthy of consideration, and we may 

 first endeavour to picture to ourselves the conditions 

 under which mountain chains arise. The floor of the 

 ocean lies at an average depth of 2,000 fathoms below the 

 land, and is maintained at a constant temperature, closely 

 approaching C., by the passage over it of cold water 

 creeping from the polar regions. The average tempera- 

 ture of the surface of the land is above zero, but we can 

 afford to disregard the difference in temperature between 

 it and the ocean floor, and may take them both at zero. 

 Consider next the increase of temperature with descent, 

 which occurs beneath the continents: at a depth of 13,000 

 feet, or at the same depth as the ocean floor, a tempera- 

 ture of 87 C. will be reached on the supposition that the 

 rate of increase is 1 C. for 150 feet, while with the 

 usually accepted rate of 1 C. for 108 feet it would be 

 120 C. But at this depth the ocean floor, which is on 

 the same spherical surface, is at C. Thus surfaces of 

 equal temperature within the earth's crust will not be 



