MISCELLANEOUS NOTICES. 
477 
due to such a loss of its original heat by long-continued radiation into the sur- 
rounding space, that from having been wholly gaseous, then fluid and gaseous, - 
and subsequently solid, fluid, and gaseous, the surface at last became so 
i reduced in temperature, and so little affected by the remaining internal 
I heat, as to have its temperature chiefly regulated by the sun, there must 
have been a time when solid rock was first formed, and also a time when 
heated fluids rested upon it. The latter would be conditions highly 
favourable to the production of crystalline substances, and the state of the 
earth's surface would then be so totally different from that which now 
exists, that mineral matter, even abraded from any part of the earth's crust 
which may have been solid, would be placed under very different condi- 
tions at difi'erent periods. We could scarcely expect that there would not 
be a mass of crystalline rocks produced at first, which, however they may 
vary in minor points, should still preserve a general character and aspect, 
the result of the first changes of fluid into solid matter, crystalline and 
subcrystalline substances prevailing, intermingled with detrital portions 
of the same substances, abraded by the movements of the heated and first 
formed aqueous fluids." Although the language is somewhat indefinite, 
the igneous theory is shadowed forth in it, and this quotation may be con- 
sidered as the text of Mr. Macfarlane's present Essay, in which he main- 
tains that there is every appearance of reason for considering that the 
primitive Gneiss formation constitutes the first solidified crust of the origi- 
nally-fused globe, and that the crystalline and subcrystalline rocks of the 
Primitive Slate formation are the products of a peculiar transition period, 
during which aqueous fluids gradually accumulated on the surface, and the 
latter attained a temperature approaching somewhat to that of the present 
I day. This number also contains a short but able article by Mr. T. Sterry 
I Hunt, " On the Earth's Climate in Paleozoic Times." Referring to our 
own T3^ndairs wonderful experiments on the relation of gases and vapours 
to radiant heat, lie shows their important bearing upon the temperature 
of the earth in former geological periods. Aqueous vapour, like a cov^ering 
of glass, allows the sun's rays to reach the earth, but prevents, to a great 
extent, the loss by radiation of the heat thus communicated: — "When 
however the supply of heat from the sun is interrupted during long nights, 
the radiation which goes on into space causes the precipitation of a great 
part of the watery vapour from the air, and the earth, thus deprived of this 
protecting shield, becomes more and more rapidly cooled. If now we 
could suppose the atmosphere to be mingled with some permanent gas, 
which should possess an absorptive power like that of the vapour of water, 
this cooling process would be in a great measure arrested, and an effect 
■would be produced similar to that of a screen of glass ; which keeps up the 
temperature directl}" beneath it by preventing the escape of radiant heat, 
and indirectly by hindering the condensation of the aqueous vapour in the 
air confined beneath. Now we have only to bear in mind that there are 
the best of reasons for believing that during the earlier geological periods, 
all the carbon since deposited in the forms of limestone and of mineral 
coal existed in the atmosphere in the state of carbonic acid, and we see at 
once an agency which must have aided greatly to produce the elevated 
temperature that prevailed at the earth's surface in former geological 
periods. Without doubt, the great extent of sea, and the absence or rarity 
of high mountains, contributed much towards the mild climate of the Car- 
i boniferous age, for example, when a vegetation as luxuriant as that now 
found in the tropics flourished within the frigid zones ; but to these causes 
must be added the influence of the whole of the carbon which was after- 
wards condensed in the form of coal and carbonate of lime, and \^hich 
