and the Earth's Thermal History. 

 Table III. 



67 



Of these results only that for basic rocks gives a temperature of 

 the order required to explain the facts of vulcanism. The two others 

 are manifestly absurd. Even if they be corrected to take into account 

 the heat represented by the ascent of igneous materials through the 

 crust (including the heat loss from volcanoes, fumaroles, solfataras, 

 geysers, hot-springs, etc.) and a liberal allowance of 20 per cent of 

 the earth's total loss of heat be assigned to this portion of it, the 

 calculated temperature would still be far removed from the high 

 values so forcibly demonstrated by vulcanism. The figures obtained 

 for acid and basic rocks point to the way of reconciliation. Equation 4 

 indicates that A must decrease in depth if adequate temperatures are 

 to be reached. That is to say, on the continents, where acid rocks 

 predominate, the radio-activity of the rocks must decrease in depth ; 

 and a few trial calculations show that the decrease must be very rapid 

 in order to allow the radio-active elements to persist to sufficient 

 depths to give the requisite temperatures. This kind of decrease 

 implies, as Table I clearly shows, a corresponding decrease in the 

 acidity of the rocks and an increase in their density. The passage, 

 at suitable depths, from granite to rocks of gabbroid composition would 

 adequately accommodate the requirements. 



Even in the case of average rock material the same argument holds. 

 Under a continent of average composition the rocks in depth must 

 soon give place to basic types. Briefly, the argument is that volcanic 

 temperatures demand a rapid decrease of radio-activity with depth, 

 and therefore a corresponding change in the composition of the igneous 

 rocks which carry the radio-active elements. 



The temperature arrived at for the basic rocks is satisfactory as it 

 stands, but it has no meaning unless it be admitted — (1) that under 

 the deeper and more permanent parts of the oceans the rocks are 

 predominantly basic; (2) that the heat lost from the sub-oceanic 

 rocks is about the same per unit area as that lost from the continents. 



The first of these assumptions is supported by gravity measure- 

 ments and the conception of isostasy, by the rocks themselves as 

 exhibited in oceanic volcanic islands, and by the physical difference 

 between sub-oceanic and sub-continental rocks which has been 

 revealed by Oldham's interpretation of earthquake records. The 

 second assumption is supported by the fact that the temperature 

 gradients near the ocean borders are steeper than those well within 

 the continents. The amount of heat issuing from the earth is 

 proportional to the product of conductivity and gradient. Thus, if 



