440 E. J. Garwood — Concretions in the Magnesian Limestone. 



allowed to stand, it rapidly crystallizes, forming a mass of small 

 crystals. If, however, the solution had been previously diluted, 

 fewer and larger crystals would have been formed, and this dilution 

 might have been carried to a point, after which further dilution 

 would have prevented crystallization from taking place altogether. 



That the presence of magnesia is the primary cause of the forma- 

 tion of the concretions is borne out by the fact that they are confined 

 to this formation, although there are plenty of other limestones in 

 the geological sequence in which they should otherwise have occurred. 

 We may remark in passing that no argument in favour of the 

 ' Stalactitic ' theory can limit the application of that theory to these 

 particular beds, for the character of the deposit into which the lime 

 is supposed to have been introduced could have no influence what- 

 ever in causing that lime to be dissolved from the beds above. 



The second part of the question, namely, the process by which 

 the particles of lime were conveyed from their original positions in 

 the deposit, and were aggregated round their respective nuclei, is 

 a very difficult one to answer, and we do not appear to have 

 advanced much towards the right understanding of this process 

 since the days when Prof. Sedgwick wrote. Broadly, the process is 

 comparable to the solution of sponge spicules in the chalk, and their 

 redeposition round nuclei in the form of flints.^ Carbonate of lime, 

 and silica are, however, different material. What do we know 

 about the pectous and colloid forms of carbonate of lime ? It is 

 true that the aragonite forming the shells of moUusca is an animal 

 product, and may perhaps be analogous to the form of silica known 

 as organic ; and we know that it is frequently converted into calcite 

 in fossil shells, showing that a rearrangement of the molecules of 

 carbonate of lime does take place after the entombment of the shell. 

 The carbonic acid and other gases given off from the decomposing 

 organic matter in the deposit, and existing under considerable 

 pressure, have probably played an important part in maintaining 

 the particles of carbonate of lime in a state of molecular suspension, 

 in which condition they were able to migrate to the nucleus which 

 was irresistibly attracting them. Whether this state was one of 

 true solution as usually understood, or whether it was one analogous 

 to the colloid condition of silica, it is impossible to say. Recent 

 experiments on the combination of solids under pressure, and the 

 movement of the individual molecules of a dry mixture, show that 

 more rearrangement probably takes place amongst the particles of 

 an unconsolidated deposit than we are accustomed to imagine. 



1 "W. J. Sollas, Ann. and Mag. of Nat. Hist. 5th series, vol. vi. 



