Geological Society. 221 



may be deposited so as to present, from the first, inclined planes, and 

 that we should therefore be cautious in referring to subsequent dis- 

 placement all deviations from a horizontal level : he forms an esti- 

 mate of the Destruction of Land by Breakers, of the wear and tear 

 of Running Waters, of the transport of detritus by Rivers. 



The mean Density of the crust of the earth is usually reckoned at 

 2*5. From a reference to the lists, which the author has drawn out, 

 of the specific gravities of many rocks, of the various simple minerals 

 which enter into their composition, and of certain recent shells, it 

 would seem that 26 would be nearer the truth. 



The Author investigates the Chemical Changes which Rocks undergo 

 after their formation, and the subject of Concretions, such as Ludus 

 Helmontii, &c. He remarks on the importance of attending to the 

 Cleavage of Rocks, whether igneous or aqueous, and their Transforma- 

 tion : he considers the great Breaks of the Surface in reference to the 

 effect which would result from its gradual cooling; and, from the con- 

 tortions and Fractures cf Mountain Chains, infers the Intensity of the 

 forces that have acted upon them : he shows that certain Thermal 

 Springs maybe occasioned by the Condensation of volcanic discharges 

 of gas and vapour, and ascribes the Uniformity of their Temperature 

 to the Constancy of such condensation : he then treats of Volcanic 

 Action and the gradual Rise of large tracts of Land. 



When explaining the Formation of Valleys, Mr. De la Beche con- 

 tends that the M Bursting of Lakes," as it has been termed, could not 

 take place in the way supposed. The Area, comprised within sound- 

 ings, that is, within the 100 fathom line, round the British islands, is 

 delineated on a map, in order to show, that within that area at least, 

 no Valleys are produced by Tides and Currents j whence it would fol- 

 low, that such effects cannot be satisfactorily referred to such causes. 

 Under the head of Faults, which are treated of at some length, the 

 author shows with what facility " Craters of Elevation" may be formed, 

 and expresses surprise that so much discussion should have taken 

 place on so simple a case ; he sees no difference between many Me- 

 talliferous Lodes and Faults. 



The subject of Organic Remains is next investigated, and the Modi- 

 fication produced by various physical causes on the distribution of Life, 

 particularly Animal Life in the sea. Diagrams are given to show that 

 Shells, contemporaneously enveloped by rocks now forming, would 

 probably not be of the same Species, even under the same parallels of 

 latitude ; but that the Species would be determined in some measure 

 by the relative depth of the water at different places, on the influx of 

 rivers, and other causes. Attention is particularly called to the man- 

 ner in which Organic Remains may be entombed in a series of deposits 

 along a gradually rising coast. 



Under the head of Mineralization of Organic Remains, the Author 

 shows that these bodies are not merely changed in character but in 

 reality. One substance being substituted for another, a cast for an 

 original, the Change varies in these bodies in proportion to their re- 

 spective Solubility. Carbonate of lime being more soluble than phos- 

 phate, shells change much more rapidly than bones. Silica in shells 



