and the Origin of Mountain Ranges. 207 



Let us assume, as Lord Kelvin does, that the average rate of 

 augmentation of temperature downwards is now 1° F. in 50 feet. 

 In the early stages of the earth's geological history, as we have seen, 

 it must have increased very much more quickly ; but we will deal 

 with the problem under present average conditions, which is stating 

 the case in its worst aspect for my theory. Also, it will be con- 

 venient to assume that the sediments are being laid down in a 

 shallow trough or basin of the ocean bordering a great continent, 

 and that the waste of a very great area of land is being deposited 

 off shore in waters of varying depths. It is a pretty well established 

 fact that the area of deposition in such a case is considerably smaller 

 than the area of denudation. If we take the waste of the continental 

 land at a mean of 1 foot in 3000 years, the rate of deposition on the 

 average will be as much more as the basin of deposition is smaller 

 than the area of denudation, so if it be one-third, the mean rate of 

 deposition will be 1 foot in each thousand years. I merely give 

 these figures just to put the matter in somewhat like true per- 

 spective. 



The sediment will all be laid down at an uniform temperature, say, 

 of 50° ; but as the matter accumulates a temperature grade will be 

 established in course of time corresponding with the temperature 

 grade due to the secular cooling of the earth as a whole ; in other 

 words, sediments, by obstructing the outward flow of heat, will 

 augment the temperature downwards, so that the rocks of the ocean 

 floor .upon which the sediments rest will be at a temperature due to 

 that depth. 



If a mile of sediments have accumulated — and this is a mere trifle 

 in the sedimentary crust of the globe — and the temperature gradient 

 is 1° for every 50 feet, the rocks originally having a temperature 

 of 50° F. will be raised to 155 "6°, and this increase of temperature 

 will eventually affect in a diminishing degree the whole of the 

 rocks underlying the area of deposition. This effect is described in 

 precise language by saying that the deposition of sediment raises 

 the " isogeotherms " or isothermal surfaces in the underlying part 

 of the earth's crust. We have assumed for the sake of simplicity, 

 that the sediments deposited have the same mean conductivity as 

 has been inferred for the whole of the earth's crust. It is possible 

 that the mean conductivity of the sediments may be less, as ex- 

 perimental proof has been given that rocks are better conductors 

 along the bedding planes than across them, and underlying rocks 

 are often on edge.^ If the sediments are worse conductors the 

 increase of heat downwards would be more rapid, if better con- 

 ductors the rate would be less. 



It will now be seen that the laying down of sediment conserves the 

 heat of the underlying part of the globe, so that the rate of waste 

 or secular cooling is not so high under sedimentary areas as under 



^ Prof. Joseph Prestwich observes (Proc. Eoyal See. 1866): "With respect to 

 the possibility of change in the thermometric gradient at great depths, it is known 

 that the conductivity of wrought iron diminishes as the temperature increases." 

 From this he infers that the thermometric gradient may increase at great depths. 



