539 



authors, and the magnesite of others."* Although no doubt can be 

 entertained that a deposit of the kind referred to does occur, as stated, 

 no one is warranted in assuming that it is in its original condition. 

 There are strong grounds for believing, as is the case with the gypseous 

 and dolomitic beds associated with it, that the mixed deposit under 

 consideration was originally differently constituted. Much uncertainty 

 generally prevails among geologists as to the origin of magnesian lime- 

 stones, which equally applies to the rocks composed of hydro -magnesian 

 silicates : the sepiolite of France and Spain is a case in point. According to 

 Dr. Sullivan and Prof. J. P. O'Reilly, f there occurs extensively in the 

 basin of the Tagus and Duero a white or greyish-white dolomitic lime- 

 stone, "containing from 25 to 40 per cent, of hydrated silica;" andin parts 

 of the same region, especially at Vallecas, near Madrid, there also occurs 

 a " hydrated silicate of magnesia, 2MgO, 3 Si0 2 -{-2HO, accompanied 

 by halb-opal, a variety of silica, chalcedony, and hornstone in a marl bed." 

 The facts stated " suggest a connexion between these minerals and 

 the siliceous dolomite ;" and the " connexion is made more probable by 

 the occurrence of fossils of Helix, which, with many species of fresh water 

 shells, are abundant in the lacustrine limestone of the central plateau, 

 converted into meerschaum" (sepiolite). "Another fact which favours 

 it is the occurrence of pseudomorphs of meerschaum after calcite" (or 

 more probably dolomite, as suspected by the authors) " in druses of the 

 former." Sullivan and O'Reilly explain the origin of the sepiolite from 

 the siliceous dolomite by the action on the latter of water holding 

 carbonic acid in solution : assuming this, the whole of the lime would 

 be gradually removed, while the magnesia, slowly combining with the 

 silica, would be converted into sepiolite ; and any excess of silica would 

 be converted into halb-opal. { 



The resemblance between the lacustrine deposits of Spain and those 

 of France is so strong in many respects as to lead Professors Sullivan 

 and O'Reilly to suggest a common origin for both. Agreeing with 

 them, we refuse to accept the second; evidence as a case in point. 



Third. The mineral, neolite, which is deposited in some mines in 

 Arendahl, may be received as showing that an alumino -magnesian 



* " Geology of Canada, 1863," p. 577. 



f " Atlantis," vol. iv., p. 315, and "Notes on Spanish Geology," p. 171, 1863. 



J The ex-President of the Geological Society, Mr. Warrington W. Smyth, in 

 stating " there can be no doubt that the ' Vallecas meerschaum' has been produced by 

 silica, probably hydrated, brought into contact with carbonate of lime and magnesia, 

 held in solution in water by carbonic acid" (see Anniversary Address, " Quarterly 

 Journal of Geological Society," vol. xxiii., p. xvi.), has been misled into giving coun- 

 tenance to Sterry Hunt's view by the " well-known" laboratory fact, noticed by Dr. 

 Sullivan and Professor O'Reilly, as showing the reaction between these bodies. In 

 omitting, which we believe to be altogether an inadvertency, the mode these authors 

 have suggested for the origin of the Vallecas sepiolite — that is, from an already-formed 

 dolomite — the pseudomorphic view they have put forward has been altogether lost sight 

 of; and a totally different one— inapplicable to the case except as an illustration, and 

 involving a contemporaneous precipitation from a chemical solution — put in its place ! 



