24 Report of Dr. Turner's Lecture 



tained in the sap of certain plants, if not in all. For it was 

 shown by the late Sir H.Davy that silex is contained in grass, 

 and in the epidermis of reeds, corn, canes, and of hollow plants 

 in general. The existence of silex in the sap of the bamboo was 

 not only attested by its flinty epidermis, but by the siliceous 

 concretions called tabasheer. Similar evidence was afforded 

 by some fossils, which contained silex in such a form as to in- 

 dicate that it was deposited from a solution. In proof of his 

 position the lecturer exhibited samples of shells having their 

 form preserved in silex, some beautiful specimens of silicified 

 coral, and a suite of chalk flints which displayed the structure 

 of sponges and other zoophytes. For the opportunity of ex- 

 hibiting such specimens he was indebted to the indulgence of 

 the President and Council of the Geological Society. Traces 

 of organization might by careful examination be so frequently 

 detected in chalk flints, that he was disposed to the opinion 

 of those geologists who considered flints in general as zoo- 

 phytes fossilized by silica. The lecturer next adverted to the 

 formation of calcedony, and showed specimens which, though 

 found in igneous rocks, had their aqueous origin clearly esta- 

 blished by the stalactitic form which they possessed. Similar 

 masses of calcedony existed in some flints, and passed into the 

 substance of flint by insensible gradations. The hollow balls 

 of crystals, called geodes, afforded similar testimony, by pre- 

 senting both calcedony and rock crystal under circumstances 

 indicative of pre-existing solution. 



The fact being established, — that siliceous minerals are fre- 

 quently formed from aqueous solution, the lecturer went on 

 to state the principles by which he thought the solution of si- 

 liceous matter, and its subsequent deposition, might be ex- 

 plained. The first observation he would make related to the 

 meaning of the term insoluble. Chemists, he said, apply it to 

 substances which are not found to lose an appreciable weight 

 when subjected to the action of water. It was not affirmed 

 that absolutely nothing was dissolved in such cases, but that 

 the quantities were too small to be appreciated. This was 

 true even of one of the most insoluble substances known to 

 chemists; namely, sulphate of baryta. But though the weight 

 of such bodies was not perceptibly diminished by trials con- 

 ducted in the laboratory, during a short interval of time, and 

 with small quantities of water, the effect of the same opera- 

 tion, as performed on the great scale in the mineral kingdom, 

 during hundreds and thousands of years, and with unlimited 

 quantities of the menstruum, might be, and doubtless was, 

 very different. It was not necessary, however, to have re- 



