11 



to, but not directly over, the first one ; or droppings took place at the 

 same time from two points, so close as to produce twin cups. 



The rounded particles formed by the droppings acted as the nuclei 

 around which deposits took place, so that they often became enlarged 

 from the size of a peppercorn to that of bullets, or larger. "When a num- 

 ber of these got imbedded in the soft mineral mud, a pisolithic mass 

 was formed. Some of the balls, however, contain so large a nucleus of 

 the translucent opal-like compounds of silicate and carbonate of zinc, to 

 be described further on, that we must suppose them to have been formed 

 by the falling of large drops of water holding silicates in solution into 

 a solution of hydrocarbonate of zinc. 



The fossil bones lay on this floor, partially or wholly enveloped in 

 the hydrocarbonate. The greater part of the collection has been 

 transferred to some Spanish museum, so that, for the present, we can- 

 not give any particular account of them. A few fragments, however, 

 having fortunately come into our hands, an opportunity was afi'orded 

 of making a chemical examination of them, with a view of determining 

 how far a substitution of lime by zinc took place. The results will be 

 found further on. 



The under side of a piece of the floor, in which a bone completely enve- 

 loped in hydrocarbonate was partially buried, was composed of a kind of 

 conglomerate of flattened, and more or less rounded, fragments of hydro- 

 carbonate of zinc, evidently the result of the action of running wate 

 They were, in fact, the pebbles of a stream upon which the bones rested, 

 and which were cemented by hydrocarbonate, and then covered over, and 

 the bones more or less buried in the successive layers of hydrocarbonate 

 of zinc deposited in comparatively still water. 



The hydrocarbonate of zinc is found in compact earthy masses of a 

 pure white colour, or slightly coloured brown by organic matter, 

 and more or less distinctly laminated, as a friable bergmehl-like sinter, 

 as stalactites, concretionary nodules, pisolithic masses, &c. It is usu- 

 ally associated with silicate of zinc, which is found coating it in small 

 crystals, or in layers composed of colourless translucent fibrous crys- 

 tals. Sometimes these layers alternate with the hydrocarbonate ; 

 even when the fibrous silicate occurs in concretionary masses of consi- 

 derable thickness, each layer appears to be separated by an extremely 

 thin opaque parting of hydrocarbonate of zinc. Layers of hydrocar- 

 bonate are often found having the fibrous structure of the silicate, 

 but containing no silica. They may possibly be the result of pseudo- 

 morphic action, and consequently to be regarded as pseudomorphic 

 hydrocarbonate after fibrous hydrated silicate of zinc. This intimate 

 association of hydrated silicate of zinc and hydrocarbonate of zinc 

 extends much further than mere mechanical associations ; for in the 

 balls already mentioned we shall find examples of combinations of the 

 two in various proportions, and even the pure fibrous silicate will be 

 shown to contain carbonic acid. 



The preceding observations indicate the chronological order in which 

 the different kinds of zinc ores in the province of Santander have been 



