380 PROFESSOE J. W. JUDD. 



found that a specimen of Lithoth amnion, which contained about 11 per cent, ol 

 magnesium carbonate, when treated with acetic acid till about 60 per cent, of the 

 mineral matter was removed, had become enriched with magnesium carbonate to the 

 extent of 20 per cent., OAvingto this more rapid leaching out of the calcium carbonate. 

 In the same way it was found by Hogbom that a coarse lagoon-mud treated with 

 acetic acid, till 80 per cent, of the material was removed in solution, gave the 

 following result. Before treatment the percentage of magnesia present was 

 1"79 per cent, and afterwards as high as 4*4 per cent. 



Indirectly, however, we have abundant evidence that under the solvent action of 

 carbon dioxide a leaching out process takes place in mixtures of the calcium and 

 magnesium carbonates, and the former being dissolved more rapidly than the latter, 

 the magnesium percentage in the residue constantly rises. 



In connexion with this point the very valuable Memoir of Professor E. J. Garwood, 

 ' On the Origin and Mode of Formation of the Concretions in the Magnesian 

 Limestone of Durham,'''^ a Memoir which has perhaps not received the attention 

 which it deserves, has a very important bearing on the subject under discussion. 

 Professor Garwood shows that the curious concretions so well-known for their 

 simulation of organic form, are the result of the solvent action of water containing 

 carbon dioxide upon a mass containing various proportions of the calcium and 

 magnesium carbonates, up to 46 per cent, of the latter salt. The concretions, however, 

 consist almost entirely of calcium carbonate, the proportion of magnesium carbonate 

 being only from 1'5 to 4 per cent. It is evident, therefore, that in the solvent action it 

 is the calcium salt which is taken up almost to the exclusion of the magnesium salt. 



Hogbom's analysis of stalactites taken from caves in the coral rocks of the 

 Bermudas entirely confirm the results and reasoning of Professor Garwood. He 

 found that these stalactites — clearly formed by the percolation of water with carbon 

 dioxide through the coral-rock — contained only "18 to '68 per cent, of magnesium 

 carbonate, while the coral-rock itself contains about five times that percentage of the 

 magnesium carbonate, t 



Hogbom's researches on the composition of the marine laminated marls of Sweden, 

 which are deposited from water (formed by the melting of the inland ice), after 

 it has flowed over the Silurian rocks of the country, are of especial interest and value 

 in connexion with this question. He shows that as we proceed farther and farther 

 from the limestone rocks the proportion of the carbonates to argillaceous matter 

 regularly diminishes. If, however, we compare the relative proportion of the calcium 

 and magnesium carbonates in the Silurian limestone with that in the sediments 

 derived from them, a very remarkable and significant difference is detected. While 

 the proportion of magnesium carbonate in the Silurian limestone is only about the 

 one-hundredth part of the calcium carbonate, the laminated marls yield from 3 per 



* 'Geological Magazine,' Dec. 3, vol. 8 (1891), pp. 433-446. 

 t Loc. cit., p. 271. 



