CHEMICAL EXAMINATION OF BOCKS AND MINERALS. 71 



alumina, protoxide of iron, silica, &c, which is given in the Tables repre- 

 senting the analyses of limestones and dolomites, conveys but little informa- 

 tion as to how they came there. On the other hand, by studying the nature 

 of the residue left by the magnesian limestone of the Permian formation, we 

 have evidence that they were formed by single deposition. Again, the residue 

 of delicate fibrous quartz which is found in the Howth dolomite, if not 

 characteristic, is at least indicative of change subsequent to its deposition, a 

 conclusion strongly supported by the cellular structure of the rock, which, 

 according to Elie de Beaumont and Morlot, affords incontestable proof of 

 alteration subsequent to the deposition of calcareous rocks. The dolomitic 

 conglomerate of Down Hill and the pseudo-dolomite belong to a different 

 class of phenomena. In the former case we have a species of conglomerate 

 formed of chalk and decomposing amygdaloidal basalt. The calcareous 

 part became more or less dolomitic, crystals of bitter-spar being sometimes 

 formed ; the magnesia forming the essential part of the residue is evidently 

 the source of alteration, and accordingly varies, as is seen in the Table, and 

 sometimes even wholly disappears. The protoxide of iron also is gradually 

 removed along with the magnesia, and so completely sometimes that only a 

 siliceous skeleton remains. The previous analyses of the mass give us no 

 clue whatever to this mode of formation, and indeed do not afford any 

 evidence whatever of the difference between it and any other kind of dolo- 

 mite. The character of the residue fully explains the history of the pseudo- 

 dolomite. It consists of the relics of some felspathic rock enveloped in a 

 mass of carbonate of lime, magnesia, and iron, themselves the products of 

 decomposition of local trappean rocks. So far as the individual rocks ex- 

 amined are concerned, the results are of course new ; but the formation of 

 dolomites of the character just described has been long since known and their 

 relationship to igneous rocks clearly indicated; I do not therefore bring 

 forward the preceding examples because they contain any general fact 

 hitherto unknown, but because they serve to illustrate the true method which 

 should be followed in the analysis of rocks. To complete the illustration, it 

 would be necessary to contrast the results obtained by means of it, with 

 the many elaborate tables of analyses annually published, and which, so far 

 as the explanation of geological phenomena is concerned, are wholly value- 

 less, however admirable they may be as specimens of skill of the analysts in 

 separating different substances from one another. 



5. Calp and Lower Limestone Shales. — The mountain limestone dissolves 

 in acids without leaving any earthy residue ; and when the solution is filtered, 

 only a little charcoal remains on the filter. But when portions of the in- 

 termediate rocks are treated with acid, they leave residues more or less 

 abundant, consisting of sand, clay, or carbonaceous matter and iron pyrites. 

 These residues, disseminated through limestones, completely alter their litho- 

 logical appearance, and communicate to them different physical properties ; 

 in this way are formed the various limestone-shales, grits, &c. of the car- 

 boniferous formation. These calcareous substances, when digested for a 

 longer or shorter period according to circumstances, in water slightly acidified 

 by hydrochloric acid, are easily penetrated in the cold, and the whole of 

 the carbonate of lime is dissolved out. The skeletons which these different 

 deposits leave on being thus treated, indicate very clearly, as in the case of 

 the Permian magnesian limestones, some of the conditions under which the 

 rocks have been formed. 



A specimen of hydraulic limestone from the environs of Milltown, Co. of 

 Dublin, treated in the manner just described, gave — 



