190 E II". Skeats -The Formation of Dolomitt 



leaching as of minor importance, and emphasizes the effects of 

 marine leaching. He has studied the marine marls derived 

 from the Silurian limestones of Bothnia, and deposited in a 

 southerly direction. Analyses show that as one goes further 

 from the Silurian source, the composition of the marls pro- 

 gressively changes in two ways. Firstly, it is noticed that the 

 percentage of carbonates to argillaceous matter steadily falls in 

 a southerly direction, but in addition, when the carbonates are 

 analyzed it is found that the proportion of magnesium to cal- 

 cium carbonates steadily rises in the material deposited farther 

 and further from the parent source. This result is attributed 

 to the greater solubility of calcium carbonate as compared with 

 magnesium carbonate under the conditions of marine leaching 

 in carbonated sea water. 



Professor Judd* accepted this hypothesis of marine leaching 

 in regard to the upper part of the Funafuti bore, where at the 

 surface the limestone contains only about 2-3 per cent MgC0 3 , 

 while at 15 feet below the surface, the MgC0 3 percentage has 

 risen to over 16. Professor Judd believed that this increase 

 in Mo-OO, was not clue to the introduction of MgCo, from the 

 sea water, but to the removal of CaC() 3 by leaching in carbon- 

 ated sea water. 



In discussing the validity of this hypothesis of the formation 

 of dolomite by differential leaching of the two carbonates it is 

 important to realize that the normal fresh limestone composed 

 of calcareous organisms, such as a coral limestone, ordinarily 

 contains not more than about 1 per cent of MgC0 3 . The 

 authorf has pointed out that if the original limestone contains 

 1 per cent of MgCO s and assuming that only the CaCO, is 

 dissolved by carbonated water, 80 per cent of the original rock 

 must be removed by solution before the MgC0 3 of the remain- 

 der reaches 5 per cent, 90 per cent must be dissolved before it 

 reaches 10 per cent, and over 93 per cent before the MgOO, 

 reaches 16 per cent. That is to say, long before the rock is 

 completely dolomitized it must be reduced to a quite structure- 

 less mass, and all traces of organisms must necessarily disap- 

 pear. This result appears to have been overlooked by the 

 authorities quoted above who have involved the leaching 

 hypothesis to account for the formation of dolomites. 



While it is true that the process of dolomitization tends to 

 obliterate the organisms formerly present in a limestone, traces 

 of their outlines in undisturbed condition can usually be seen 

 under the microscope, and in many of the dolomites of the 

 Carboniferous limestone in Britain and Ireland traces and casts 

 of organisms are commonly to be seen either in the mass or 



* Judd, Funafuti Report, London, 1904, p. 384. 

 f Skeats, Q. J. G. S., lxi, 132, 1905. 



