Hardman— 0>* tJie Carboniferous Dolomites of Ireland. 705 



LY. A CoN-TEIBUTIOJiT TO THE HiSTOEY OF DoLOMITE. ThE DoLO:MITES 



OF THE Cahbokifeeotjs Lijiestone OF Ieeland. By Edavaud T. 

 Hakbman, p. C. S., r. E. G. S. I., of the Greological »Survev of Ire- 

 land. (With Plates 41 and 42.) 



[Read May 8, 1876.] 



As Bischof well remarks, "no rock has attracted greater attention, 

 than dolomite ;" and very many theories as to its origin have been put 

 forward ; the principal of which are based on the idea of the meta- 

 morphism, in some way, of limestone rocks, varying the means of such 

 ■changes according to the views of different authors. 



(1). Von Buch's supposition involves the introduction of magnesia 

 into limestone, as the result of the eruption of volcanic rocks in the 

 neighbourhood, producing vapours of magnesic chloride.* 



(2). Haidinger suggested that the effect was produced by the 

 action of sulphate of magnesia on limestone, sulphate of lime and 

 carbonate of magnesia being formed.f But as this cannot be effected 

 in the ordinary way, it is assumed that under the influence of great 

 lieat and pressure it might take place. It will be remembered, how- 

 ever, that most dolomitic deposits of themselves utterly refute such an 

 hypothesis. 



(3). Von Morlot put forward a similar theoiy, having, as he sup- 

 posed, found that when sulphate of magnesia and carbonate of lime 

 were heated in a sealed tube to a temperature of 392° F., a double 

 carbonate of lime and magnesia was formed, together with gypsum. :]: 

 But Dr. Sterry Hunt has shown that in this case the so-called dolo- 

 mite was really but a mixture of carbonate of lime with carbonate of 

 magnesia, § nor did he find that Marignac's|| substitution of chloride of 

 magnesium for the sulphate yielded any better results. 



All these well-known theories not only presuppose in every case 

 the action of igneous rocks, and a high temperature, but also the 

 evolution of gaseous sulphuric and hydrochloric acids, in order to 

 obtain the necessary supplies of magnesia from the eruptive rocks them- 

 selves. In our present state of chemical and geological knowledge, 

 it will therefore be doing no \iolence to the scientific reputation of 

 their originators, to say that they may now be looked upon rather as 

 curiosities of geological literature, 



(4). Forchammer appears to refer the formation of dolomite to 



* See Bispliof, Chem. Gcol., vol. iii., 155 et seq. Also Chem. and Geol. Essays, 



Sterry Hunt, 44 D., &c., p. 81,— Ann. de Chem. and Phys., xxiii., 296. 



t Bischof, op. cit., p. 158. — Pogg. Ann., Ixxiv., p. 591. 



+ Bischof, loc. cit., also N. Jahrb. fiii- Min., 1847, 862. 



§ Chem. and Geol. Essays T. Sterry Hunt. 



11 For the experiment, see Bischof, vol. iii., p. 159. 



3 T2 



