PROF. VV. r. VVILLfAMSON ON M INKKALIZA riON. 191 



in which they are chiefly found are those of which Halifax. 

 Huddersfield, Ashton-under-Lyne, Oldham, and Rawtenstall 

 are the principal towns. The district had originally been dry 

 land covered with a forest vegetation, accompanied probably 

 by a rich undergrowth, which combined to form a layer of 

 vegetable matter. This surface sank below the sea level, and 

 was covered up by successive layers of marine sediment in 

 which numerous marine shells are still preserved, and in some 

 of which calcareous concretions are also found. The pro- 

 bability is that water containing carbonic acid has filtered down 

 through the mud containing these calcareous elements, and 

 reached the layer of vegetable matter, the tissues of the com- 

 ponent plants not having as yet perished. By the remarkable 

 process of Osmosis illustrated by the late Professor Graham, 

 the water containing lime in solution penetrated these tissues, 

 and not only filled their minutest cavities with that alkali, but 

 invested masses of them in calcareous concretions which for 

 ever prevented any of the agencies productive of decay from 

 reaching their most delicate structures. 



The elements composing these protective concretions are 

 chiefly carbonate of lime with a little magnesia and iron. 



Leaving the calcified objects, we come to those replaced by 

 Silica, flint, agate, or quartz. Silicified examples of fossil plants 

 are common enough, but in order to understand something of 

 the way in which silicification has been brought about we must 

 say a word or two explanatory of the way in which the Silicium 

 has acted. You are all aware that under certain conditions 

 Silica, or flint, is capable of being converted into a jelly-like sub- 

 stance, the hydrate of Silicium — commonly called water-glass, 

 or colloid; if a small quantity of this colloid Silica is put into 

 a large quantity of water it will dissolve, though it takes an 

 immense amount of water to dissolve a little of the mineral; 

 but if in addition you introduce some alkaline element into the 

 water the solvent power that water has over the Silica is 

 enormously increased, in which state of solution it will readily 

 penetrate the interior of complex organized bodies. It is from 

 Silica in this state, in all probability, that we obtain the well, 

 known numerous examples of silicified woods that exist. Some 

 of you are probably familiar with the fact that at Autun and 

 Saint Croix in France we find fossil plants are so silicified. 



