May 271 d, 1922.] Proceedings. xxv. 



We will now consider the evidfence of density as to the struc- 

 ture of the earth below the crust. 



The density of the crust is 2.5 



„ Siliceous rocks 2.4—2.7 



Basic 2.8—3.7 



The earth ... 5-5— 5-6 



This high density of the earth may be accounted for by the 

 presence of the heavy elements, and more particularly of the 

 heavy metals in the central parts such as iron, gold, platinum, 

 iridium, etc. It may be explained by the view that the earth 

 consists of a metallic centre surrounded b3^ heavy bases arranged 

 according to their density in a series of zones ending in the 

 crust. 



I now turn to the evidence of temperature as to structure. 

 The volcanoes and hot springs prove that there is great local 

 heat at great depths below the surface, and the experience of 

 miners everj^where establishes the increase of heat as the depth 

 increases, the variation being due to the varying conductivity 

 of the rock. If we follow Snell in taking the average increase 

 in Europe to be one degree rise for 65 feet of depth, the tempera- 

 ture at a depth of 34 miles will be no less than 2786 degrees 

 Fahr., sufficient to melt most of the elements. Hence the idea 

 that the crust is floating on a molten ocean. But if the heat at 

 great depths is enormous so is the pressure which raises the 

 melting point— as is well known in steel works. If the tempera- 

 ture once masters the pressure there will be fluidity. If the 

 pressure preponderates, there will be solidity. As the relation 

 of heat to pressure at great depths is unknown we cannot be 

 certain whether the crust is based on fluid or solid zones of tlie 

 two magmas. 



According to Lord Kelvin the earth as a whole is an elastic 

 solid like an equal sphere of glass or steel, while Hopkins takes 

 it to be solid to a depth of 800 to 1,000 miles from the surface. 

 In my opinion it is probably a solid with fluid areas of molteu 

 rock where the temperature is high enough to overcome the 

 pressure. 



The crust follows the contraction of the cooling earth, and as 

 it has to occupy a smaller space is subjected to enormous lateral 

 pressure which is indicated by the folds and the faults. The 

 latter divide the rocks into wedges, one series with their bases 

 downwards and the other with their bases upwards. The move- 

 ment as a whole is vertical, the greatest lateral displacement 

 on record being between 5 and 6 miles in the Highlands of 

 Scotland. The existing land is an area of elevation, and the 



