FROF. W. C. WILLIAMSON ON MINERALIZATION. 187 



Now at the first glance, this is a very simple question, involving 

 very few and easily understood elements. Of course, there 

 are certain examples of animal life, and even some cases 

 in all probability of plants too, in which we find nothing but 

 protoplasm, which is a wholly soft tissue ; then there are others 

 in which — like shells and crustaceans — there are various forms 

 of a hardened external integument or shell — whilst in a third 

 group you have an internal skeleton clothed externally with soft 

 tissues. Now, as a general rule, when we speak of fossils we 

 refer to the hardened structures — we speak of the shells of the 

 Mollusca, the hardened chitinous skin of Insects, or the internal 

 skeletons of the higher forms of life. 



Let lis take a very simple case ; here (drawing) we have an 

 organic structure imbedded in soft mud, but which mud under 

 various subsequent processes of nature became hardened, 

 excluding the air from the imbedded object. In fact, it became 

 an organism enclosed in a mould, from the interior of which 

 the atmosphere was excluded. At a still later period this shell 

 was exposed to the action of destructive agencies, chiefly in the 

 shape of acids held in solution in the water of the sea. Sup- 

 posing that this shell was composed of carbonate of lime and 

 buried in the bed of the sea the superincumbent water contain- 

 ing carbonic acid would filter through the mud, and reaching 

 the shell would dissolve out the lime and leave a defined cavity 

 behind it. Sometimes this cavity remains unoccupied. At the 

 Royal Society last night some interesting cases of this kind 

 were brought before us. Some skulls and other bones of some 

 Reptilian vertebrates had been thus imbedded, but of which not 

 a trace of the animal substance remained. But by filling these 

 cavities with some plastic material, I think gutta percha, Mr. 

 Newton, of Jermyn Street, was able to represent to us the 

 reconstruction of such remains of these animals as had originally 

 filled these cavities. 



It is important at this stage of our inquiry to note some of 

 the facts essential to our comprehension of what took place. In 

 the simpler case already referred to, an organic, calcareous 

 structure disappears, and is merely replaced by the same cal- 

 careous material in an inorganic state. It is known to all 

 familiar with the simplest elements of chemistry that water 

 containing certain proportions of carbonic acid is capable of 



