250 Report ofDr Turner's Lecture on 



mis, but by the siliceous concretions called tahasheer. Si- 

 milar evidence was afforded by some fossils, which contained 

 silex in such a form as to indicate 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 exhibiting such specimens, he was in- 

 debted 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 zoophytes fossilized by silica. The lecturer 

 next adverted to the formation of calcedony, and shewed speci- 

 mens which, though found in igneous rocks, had their aqueous 

 origin clearly established 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 testi- 

 mony, by presenting 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 silice- 

 ous matter, and its subsequent deposition, might be explained. 

 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 any appreciable weight when sub- 

 jected to the action of water. It was not afiirmed -that abso- 

 lutely nothing was dissolved in such cases, but that the quantities 

 were too small to be appreciated. This was true even of the most 

 insoluble substance known to chemists; namely sulphate of baryta. 

 But though the weight of such bodies was not perceptibly di- 

 minished by trials conducted in the laboratory during a short 

 interval of time, and with small quantities of water, the effect of 

 the same operation, as performed on a great scale in the mine- 

 ral kingdom, during hundreds and thousands of years, and 

 with unhmited quantities of the menstruum, might be, and 



