Revieivs — LyeWs Principles of Geology. 121 



derived from the study of the Secondary and Primary fossiliferous 

 formations, has been added. In it the author discusses the theory of 

 an excess of carbonic acid in the air during the Coal-period. He 

 regards as delusive, inferences deduced from the fact, that there is ten 

 times more carbon in a solid form in the ancient Coal-measures than 

 is now contained in the atmosphere. The atmosphere now 

 receives large supplies of carbonic acid, by gaseous emanations from 

 the interior of the earth ; but, wherever peat is forming, the process 

 is seen by which carbon is first extracted, by the powers of vegetation, 

 from the atmosphere, Mr. Darwin attributes the small quantity of 

 peat formed in parts of South America to the absence of species of 

 plants fitted for its production. The abundance of coal, therefore, 

 in certain districts may have arisen from the peculiarity of the vege- 

 tation, and of a climate which retarded decomposition, rather than 

 from any peculiarity in the atmosphere which enveloped the globe in 

 the Carboniferous period. 



In a special chapter the author has considered how far former 

 vicissitudes in climate may have been influenced by astronomical 

 changes, such as variations in the eccentricity of the earth's orbit ; 

 changes in the obliquity of the ecliptic, and different phases of the 

 precession of the equinoxes. Mr. CroU's suggestion as to the pro- 

 bable effects of a large eccentricity in producing glacial epochs is 

 fully discussed, and the question is entertained whether geological 

 dates may be obtained by reference to the combined effect of astro- 

 nomical and geographical causes. 



In the chapter on the phenomena of Springs, the author notices 

 the discovery of live fish in some artesian wells, sunk in the Sahara. 

 They were iDrought from a depth of 175 feet, and were not blind, 

 like those of Adelsberg, but had perfect eyes. 



The antiquity of the delta and alluvial plain of the Mississippi is 

 discussed, with reference to new facts brought to light during the 

 survey of Messrs. Humphreys and Abbot. Sir Charles had esti- 

 mated that the accumulation of the whole delta-deposits must have 

 taken 67,000 years ; but the former gentlemen, in the course of 

 their survey, came to the conclusion that the quantity of water 

 annually discharged by the Mississippi, had been greatly underrated ; 

 consequently the number of years required for the growth of the 

 whole delta would be reduced to about one-half, or to about 33,500 

 years. In the same chapter are discussed the researches of Mr. 

 W. H. Bates and Professor Agassiz on the delta of the Amazons, and 

 of Mr. Fergusson on that of the Ganges. 



A chapter on the action of tides and currents, contains an account 

 of Captain Spratt's observations in the Mediterranean. His survey 

 shows how different parts of an inland sea, and the adjoining ocean, 

 may have different temperatures at a moderate distance from the 

 surface, in consequence of submarine barriers. The range of aquatic 

 species inhabiting the waters at various depths must evidently be in 

 no small degree dependent on such continuous submarine ridges. 



An illustration of the stone-capped earth-pillars of Botzen, in the 

 Tyrol, adorns the cover. It is from a drawing by Sir John Herschel, 



