462 Transactions. — Geology. 



at very different periods of time. The further they are removed from the 

 surface the smaller is this difference between the extremes. In the latitude 

 of our temperate zone (between 48° and 52°) the stratum of invariable 

 temperature is at a depth of from 59 to 64 feet, and at half that depth the 

 oscillations of the thermometer, from the influence of the seasons, scarcely 

 amount to half a degree. In tropical climates this invariable stratum is 

 only one foot below the surface, and this part has ingeniously been made 

 use of by Boursingault to obtain a convenient, and, as he believes, .certain 

 determination of the mean temperature of the air of different places. The 

 mean temperature of the air, at a fixed point, or at a group of contiguous 

 points on the surface, is, to a certain degree, the fundamental element of 

 the climate and agricultural relations of a district, but the mean tempera- 

 ture of the whole surface is very different from that of the globe itself." 

 We have no data for determining the depth at which the stratum of 

 invariable ternperature lies within the arctic regions, but looking to the 

 increase which takes place between that at which it is found within the 

 tropics, and that at which it occurs some 20° further north, we may assume 

 it to lie at a depth of little under 200 feet in the former region. I am not 

 aware of the mean temperature of the air in the arctic regions, but it must 

 be so low as to be absolutely antagonistic to all but the most stunted and 

 hardy forms of vegetable life. 



It. must not be supposed, however, from what I have already said, that 

 the supposed gradual diminution of the primitive heat of the globe has not 

 been resorted to by geologists to account for alterations in climate. This is 

 not the case, but, unfortunately for the earlier propounders of the theory, 

 the condition of our knowledge did not afford them sufficient evidence in 

 support of it, and, indeed, it is only within the last few years that the 

 investigations of physicists have supplied grounds which would justify 

 the proposition. The authority of Sir Charles Lyel], which was arrayed 

 against it, tended moreover, to check further investigation, but although 

 (as I before observed) I may be treated as presumptuous in endeavouring to 

 set up this theory in opposition to his views, I feel that recent discoveries 

 justify further discussion on the subject. 



I now propose to consider briefly the nature of the surface conditions of 

 our globe after the condensation of the nebular matter had been completed. 

 We have in the present surface conditions of our own satellite, some evidence 

 of what that of our globe would have been but for the presence of a con- 

 trolling element, to which I shall hereafter allude. The researches of 

 Nasmyth and Carpenter on the moon, published in 1874, have given to the 

 world the clearest possible view of the present condition of her surface, 

 indicating, as that condition does in the most unmistakable manner, its 



