44 BOCK-METAMORPHISM. 



originally argillaceous, have passed directly from the earthy 

 condition into their present form. " A very homogeneous slate, 

 from the Villa Rota, on the Po, was found by Delesse to have a 

 composition so closely resembling serpentine, that it might be 

 regarded as schistose serpentine "*. 



But a more decisive case may be predicated of one lately made 

 known by Achiardi of Pisa, occurring at Podermo in Tuscany : 

 it consists of argillaceous schist methylosed into a green trans- 

 lucent serpentine by the action of subterraneous water holding 

 magnesia in solution, this substance having gradually replaced 

 the alumina of the schist f. Delesse, who has noticed the case, 

 mentions that he has observed a similar change at Odern in the 

 Vosges J. 



The hydrous silo-magnesian marl, sepiolyte (classified by 

 some writers with the minerals aphrodite, talc, spadaite, and 

 others, but differing from them mainly in containing more 

 water), which occurs as a Tertiary deposit in the Paris basin, 

 and at Vallecas near Madrid, is another instance of a formation 

 which, although it has undergone a chemical change , still re- 

 tains its original earthy or amorphous condition. 



Reverting to the case made known by Achiardi, it may be 

 mentioned that Bischof has shown, from the presence of chlo- 

 ride of magnesium in the water of many springs, that " the 

 formation of silicate of magnesia may take place by the action 

 of such water upon silicate of alumina in the form either of clay 

 or compound silicates " ||. 



* Bischof, op. cit. vol. ii. p. 416. 



t R. Coin. Geol. d'ltalia, Bollettino, 1876, p. 11. 



| Revue de Ge*ologie, 1878, p. 191. 



See footnote, p. 28. 



|| Chemical and Physical Geology, vol. i. p. 344, and vol. ii. p. 107. 



