Lesley.] x ^-> [Oct. 18, 1878. 



h;is left ils own record in a dolomite bed, — we might proceed one step 

 further and find in the larger differences indicia of ages of greater or less 

 volcanism. 



Dauhree in his "Synthetical Studies and Experiments on Metamorphism, 

 &c," saj-s (see Smithsonian Report for 1861, page 2G9) : 



"We know that certain dolomites result from the transformation of 

 limestone. This epigeny may be explained by the action of combinations 

 of magnesia or carbonate of lime. There is, however, nothing to prove 

 that this transformation into dolomite has always been produced by the 

 same agents, and that the dolomite of Campo-Longo, for instance, with its 

 tourmalines, corundums and various minerals, is to be assimilated with 

 the dolomite of the other parts of the Alps and of Nice, or those which are 

 near the deposits of calamine in Belgium. 



But there are dolomites, and this is the case with the greatest numbers 

 situated in regular beds, which are often horizontal, constituting very ex- 

 tensive geognostic formations. When they contain remains of testaceous 

 mollusca the shell has disappeared ; they are often crystalline and riddled 

 with holes in such a way as to suggest a substitution. It is possible that 

 the principal part of these last dolomites was directly precipitated. But 

 on account of the disappearance of the shells we must admit, with Elie de 

 Beaumont, that this second case allies itself with the first, by the reaction 

 which the medium has exerted on the matter precipitated, a reaction of such 

 a kind that the carbonate of lime has disappeared. Indeed we notice that 

 pure limestone never alternates with them." 



This is certainly not the case in respect of the 115 beds of our section ; 

 for certainly the limestone beds of Table 7 (A), page 121 above, with only 

 2 p. c. carb. mag. and 1.4 p. c. insol. matter, have a right to be classed 

 with pure limestones. 



The disappearance of shells by solution is not one of the noticeable fea- 

 tures of the limestone strata under discussion in this paper ; and they do 

 not, as a rule, exhibit any cavities assignable to such a cause. They are 

 non- fossil iferous, not because of the destruction of fossils, but because of 

 the absence of large forms of life in the original sediments. 



The researches of Mr. E. T. Hardman, of the Geological Survey of Ire- 

 land, published in No. 7 of the Proceedings of the R. Irish Academy, Vol. 

 II, Ser. II, Jan. 1877, valuable as they are, give us little assistance, be- 

 cause his specimens were taken from the walls of caverns in cavernous 

 limestones, where metasomatic action was in open activity. 



In the Jahrbuch der K. K. Geol. Reichsanstalt, XXV, 1875, p. 293, 

 MM. Doelter and Homes discuss the subject and assign 1. to the slightly 

 magnesian limestones a directly organic origin ; 2. to sporadic normal dol- 

 omites a later metamorphosis by percolation ; 3. to the largest part of the 

 dolomitoid rocks an original organic origin, with subsequent change of the 

 fossils by magnesia salts during or shortly after deposition, and still later 

 local lixiviation and concentration. 



