462 Transactions.—G'eology. 
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 temperature 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 Sit Charles Lyell, 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, 
ting, as that condition dons in the most unmistakable manner, its 
