MISCELLANEOUS NOTICES. 



477 



due to such a loss of its original heat by long-continued radiation into the sur- 

 rounding space, thatfrom having been wholly gaseous, then fluid and gaseous, 

 and subsequently solid, fluid, and gaseous, the surface at last became so 

 reduced in temperature, and so little affected by the remaining internal 

 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 different 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 

 day. This number also contains a short but able article by Mr. T. S terry 

 Hunt, " On the Earth's Climate in Palaeozoic Times." Referring to our 

 own Tyndall's wonderful experiments on the relation of gases and vapours 

 to radiant heat, he shows their important bearing upon the temperature 

 of the earth in former geological periods. Aqueous vapour, like a covering 

 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 snn 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 directly 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- 

 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 which 



