188 PROF. AV. 0. WILLIAMSON ON MINF.RALIZATIOX. 



dissolving carbonate of lime. You are all familiar with that fact 

 from your knowledge of what takes place in many subterranean 

 caverns ; water filtering through cracks and crevices of cal- 

 careous strata on its way to the cavern brings along with it, 

 in solution, lime that it has picked up in the manner just 

 referred to. But when such a solution is exposed to the 

 atmosphere the water again throws down the lime so obtained. 

 On reaching the cavern and dropping from the roof it becomes 

 thus exposed. The precipitated lime now coheres to form the 

 stalactitic pendants that hang from the roof, and what remains 

 reaches the floor, where it produces the layers and pillars of 

 stalagmite, as these lower formations are called. 



Now this is precisely what takes place in many forms of 

 fossilization. The original shell had a characteristic structure 

 of its own. It was replaced by the same chemical substance, 

 but which was now in a pui-ely mineral form, whether crystal- 

 line or amorphous. But we have some cases with a yet simpler 

 form of change, where little or no destruction of the original 

 object has taken place. I daresay some of you have visited 

 the celebrated dropping well at Knareshorough, and have seen 

 the fossil wigs, birds' nests, baskets of eggs, etc., that are 

 regularly produced for sale at that place. This, however, is 

 what may be termed fossilization by investment. All the 

 objects in question retain their normal features, and have 

 practically imdergone no change beyond receiving something 

 closely resembling a coat of white- wash. Though this cannot 

 properly be called fossilization, a result not altogether foreign 

 to it occurs in Nature. Thus there are objects which are more 

 or less porous, and when solutions of lime reach such, though 

 they undergo little or even no change in their outward forms, 

 the solution penetrates their minute internal cavities and 

 canals and fills them up. This, again, is little more than 

 fossilization by investment, since it only invests the surfaces 

 of the tissues of the organism, instead of replacing them. 

 But this latter process plays a very extensive part in 

 the preservation of such forms as are of vegetable origin, to 

 some special cases of which I shall shortly call your attention, 

 and some beautiful examples of which will be shown to you at 

 the close of my observations. 



Of the examples of lime thus deposited in the interiors of 



