SOME MET A SOMATIC CHANGES IN LIMESTONES. 53 



magnesia in place of lime, and finds more plausible the sugges- 

 tion that limestones originally containing some magnesia 

 have become enriched in that constituent by the removal of 

 the lime-carbonate. There seems to be some positive 

 evidence of this process going on to some extent. Thus, 

 while in corals from Bermuda the ratio Mo-CO, : CaCCX is 



o 3 3 



always under 1 : 100, and in shells still less, the ratio in the 

 reef-rock and lagoon-sediment of the same locality is from 

 17 : 100 to 4 : 100, the higher ratios corresponding to the 

 finer sediments. The author lays stress on the finely 

 divided state of the material, allowing it to remain suspended 

 in water and subjected to the "leaching out" action (Aus- 

 laugungsprocess) for a long time. In this connection his 

 examination of the Quaternary muds of Sweden is instructive. 

 These deposits came from the melting of the ice, and doubt- 

 less derived their calcareous material from the lower Palaeo- 

 zoic limestones. The latter have M^CO, : CaCO,, on the 

 average, in the ratio 1 : 100, while in the muds themselves 

 the ratio ranges from 3 : 100 to 100 : 100 (i.e., to equality). 

 Moreover, on comparing analyses from eight localities, 

 extending from the Gefie district to south of Stockholm, it 

 is found that, as we pass southward, that is away from the 

 source of calcareous matter, while the total carbonates fall 

 steadily from 32*2 to 4*5 per cent, the ratio MgC0 3 : CaCO s 

 increases steadily from 37 : 100 to 36 : 100. Hogbom 

 cites also the analyses of deep-sea deposits given in the 

 Challenger Report, and shows that as the total carbonates 

 diminish from 87*4 to 4*1 per cent., the ratio in question 

 rises from o'8 : 100 to 105 : 100. 



It must be considered doubtful to what extent the 

 process here discussed is applicable to extensive masses of 

 dolomitic rocks. To convert in this way even a Lithotham- 

 nion-rock to a dolomite, as defined by Cotta, would require 

 the removal in solution of four-fifths of the carbonate of 

 lime, or about 74 per cent, by weight of the total material. 

 This can be conceived only in the case of such fine deposits 

 as those specially discussed by Hogbom. In dolomitised 

 shore-deposits, in which shells are sometimes bodily re- 

 placed by pure dolomite, the greater part of the magnesia 



