482 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 30, 



clay is produced by its disintegration. The colour varies in different 

 places, though in general consisting of yellow and green, imperfectly 

 mixed. The colouring matter is oxide of iron or manganese ; in 

 some cases, as much as 2 per cent, of oxide of chromium. 



The surface of the rock has frequently a polished greasy appear- 

 ance, though the form is not so regular as our " slickensides," which 

 are perfectly flat. Here the margin does not present any planes, 

 but has followed the sinuosities of the rock against which friction 

 has taken place. Its structure is so incoherent that a blow with a 

 hammer shatters a mass into small fragments; though in subterranean 

 galleries much pervaded with water it appears to acquire much 

 tenacity. Gabbro seems to me to be in many places a decomposing 

 rock. 



The topographical appearance of the serpentine-eruptions is very 

 characteristic : there is an entire absence of those undulating chains 

 or eminences, melting insensibly into one another, which enable us 

 to classify hills into groups. These rocks form dykes, but more 

 generally constitute whole hills of conical form, rising abruptly to a 

 considerable height, and terminating in rugged, sharp summits. The 

 older rocks have been much upturned and elevated, and are thrown 

 off in every direction, — the serpentine, forming the nucleus of the 

 mountains so abundant along the west coast of Tuscany, Modena, 

 and Piedmont, generally reaching the surface somewhere near the 

 centre, forming (if I may be permitted the expression) a " pericli- 

 nal" axis. 



The older rocks, nearer the focus of action, are the most disturbed. 

 No feature regarding this serpentine is more important than that of 

 its being almost invariably accompanied by rich ores of copper at its 

 junction with the metamorphosed schists or gabbro rosso. These 

 two rocks, similar in name, are entirely distinct in most other 

 respects: one is an aqueous, the other' an igneous rock. 



Many minerals are peculiar to the junction of the gabbro rosso 

 and the Miocene serpentine ; they are chiefly zeolites. The com- 

 monest is caporcianite, a white crystalline mineral, tinged with 

 pink, in structure resembling analcime. These zeolites all contain 

 magnesia. They are, — 



Magnesia 

 per cent. 



Savite, containing .... 13-50 



Schneiderite 11-03 



Picranalcime 10-25 



Picrotomsonite 6-27 



Magnesia 

 per cent. 



Portite 4-87 



Sloanite 2-67 



Humboldtite , 2-12 



Caporcianite I'll 



Miemmite (dolomite) contains 42-5 per cent, of magnesia ; " gab- 

 bro," from La Spezia, 24-4. 



Calcareous spar also occurs in limpid and extremely obtuse rhom- 

 bohedral crystals ; it probably owes its origin to the metamorphosis 

 of the limestones. I consider all these minerals to have been pro- 

 duced at the period of the intrusion of the Miocene serpentine, from 

 whence they doubtless derived their magnesia. It is also interesting 



