GEOLOGY. 273 



properties of the animal substances themselves are not without influ- 

 ence ; when of a somewhat persistent nature, they are usually petri- 

 fied by means of lime-salts, while, with unstable substances, the petri- 

 faction is effected chiefly with silicates. The authors are of the 

 opinion, that the process of petrifaction is still in operation, and in 

 support of this view bring forward a number of observations made on 

 the shores of the Mediterranean. If shells are left upon the beach, 

 they gradually undergo decomposition, but are not petrified. At a 

 certain distance from the shore, the hollow and prominent parts of the 

 surface are worn down, and at length disappear ; the sand collects 

 and hardens in the cavity of the shell, smaller shells being sometimes 

 accidentally inclosed. The calcareous mass, which, by the gradual 

 process of substitution, is ultimately precipitated throughout the whole 

 substance of the shell, forms a centre of attraction for all the salts held 

 in the surrounding water, which are accordingly deposited in a crys- 

 talline state upon both the interior and exterior of the shells, forming 

 crystalline cases of carbonate of lime often very regular in structure. 

 Thus the lime originally existing in the shells is dislodged and re- 

 placed again. All species of shells, however, do not suffer the same 

 transformation. The shells of the oyster and the pecten receive the 

 petrifying solution chiefly between their lamella, and are thus 

 strengthened and more closely assimilated to stone than in their 

 natural state; some of the shells with thin valves become coated with 

 a species of calcareous cement, which fastens them together in the 

 same \vay as is seen in primeval shells. The Ostrea cdulis is often 

 covered with a crystalline coating of calc-spar, which renders it as 

 thick as those petrified in rocky masses. When the process of petri- 

 faction is complete, no vestige of the original structure of the shell re- 

 mains. When shells, which have been thus petrified, are in contact 

 with water containing organic matter in a state of petrifaction, their 

 surfaces very often assume a black or dark-blue tint, arising from the 

 formation of sulphide of iron by the action of the reduced sulphates 

 upon the sesquioxide of iron contained in the shells. In the same 

 manner as the process of petrifaction is still in operation, the forma- 

 tion of fossiliferous sandstone also proceeds without cessation. Masses 

 of shell, more or less petrified, lying buried in the sand of the Medi- 

 terranean, are penetrated by a clay, under the influences of which 

 they are hardened, as if by Roman cement. Masses of metal, lying 

 in the water, also form nuclei, around which the bases of the salts 

 dissolved in the water are aggregated with muscle-shells, sand, and 

 the oxide of the metal, thus forming the basis of a rocky formation. 

 Jameson's Journal. 



THE DYNAMICS OF EARTHQUAKES. 



IN the transactions of the Royal Irish Academy, Mr. R. Mallet pub- 

 lishes a treatise, " On the Dynamics of Earthquakes, being an Attempt 

 to reduce their observed Phenomena to the known Laws of Wave Mo- 

 tion in Solids and Fluids." As clearly as the subject will permit, we 

 shall endeavour to give an abstract of the general theory. 



