218 



On Ophites and related Rocks. 



[June 19, 



been effected by additional water penetrating them, and flowing from 

 extraneous or foreign sources. Compared with each other, mineralised 

 rocks maybe classed as xerothermal, and methylosed as hydrothermal. 



Various evidences are adduced to show that (L) " Some ophites 

 have been originally igneous, and others sedimentary rocks," — a con- 

 clusion favouring their secondary, and consequently their methylotic 

 origin. 



(M.) " Some crystalline limestones are simply mineralised," such as 

 carrarite ; though rocks closely related to them, — viz., " Dolomites, have 

 undergone methylosis." With regard to the latter, however, the authors 

 do not accept Yon Buch's theory of dolomitisation in its general appli- 

 cation. Admitting various kinds of this phenomenon, they conceive the 

 change in certain well-known cases has been effected by the action of the 

 magnesian constituents of sea water on subjacent beds of limestone ; 

 for example, during the closing portion of the Triassic period, as 

 strongly supported by geological evidences, determined by Ramsay 

 and others, the seas in certain European regions became dried up or 

 reduced ; and their water, loaded with magnesian salts, sank through 

 the subjacent sandstones and marls into the Permian limestones, thus 

 converting them into dolomites. Irish corroborative cases are men- 

 tioned. The dolomites of the Tyrol are held to have originated in 

 the same way ; but it is admitted to be probable that the predazzite 

 of the Canzacola mountain, Val di Fassa, was dolomite that became 

 hydrated by the heated water which accompanied the eruption of the 

 immediately adjacent and overlying monzonite. " Serpentinisation 

 effected in deposits without the intervention of mineralisation " is 

 admitted in the production of the magneso-argillite at Yallecas near 

 Madrid, also of that in the Paris Basin, and other localities ; for Sullivan 

 and O'Reilly have shown that it was originally a non- magnesian deposit. 



The authors conclude by treating of (N) " The Chrono-geological 

 range of Ophites, &c, and the age of their methylosis." Offering 

 merely possible suggestions as to the age in which this phenomenon 

 took place in what may be regarded as the oldest ophites (as the 

 subject is beset with considerable difficulties), instances referable 

 to secondary periods, as the dolomites and serpentine rocks of the 

 Tyrol, &c, are briefly noticed; but they refer more confidently to the 

 methylosed euphotides, &c, of Northern and Central Italy, which, 

 having burst through Cretaceous limestone (alberese), Eocene sand- 

 stones and schists, have incontestably produced gabbro verde during late 

 Tertiary ages. Moreover, it would appear, from the discoveries of 

 Achiardi, that argillaceous schists, in Tuscany, are now being serpen- 

 tinised by the action of magnesiated water. And, taking the wide 

 range of evidences, which have been adduced, into consideration, it 

 can scarcely be doubted that the same process is still in operation in 

 deep-seated rocks, permeated by heated waters. 



