254 Report of Dr Turner's Lecture on 



operations in the laboratory, was deposited in a gelatinous form, 

 hardening gradually by evaporation, and the cohesive attraction 

 of its particles. The regularly disposed lines which were so 

 beautifully displayed in some varieties of calcedony, seemed 

 owing to successive deposition; one layer succeeding another, 

 each assuming the form and irregularities of the preceding, and 

 differing in tint, according to the absence or presence of small 

 varying quantities of foreign matter, such as iron and manga- 

 nese. In the case of flint, it was necessary, he said, to account 

 for that remarkable tendency which silica possessed, to occupy 

 the place of organic matter, as exemplified by the specimens of 

 flint, silicified wood and coral, on the lecture-table. This phe- 

 nomenon, the lecturer thought, might be explained on the prin- 

 ciples which had been developed that evening. 



Siliceous solutions, infiltrating through organic masses in 

 progress of decay, might readily be decomposed by the affinity 

 of gases, or other compounds, generated during slow putrefac- 

 tion, either for the silica itself, or for its solvent. In either 

 case a deposit of silex would result. Consistently with this view, 

 it was well known that flints contained traces of bitumen, or 

 some similar substance of organic origin. To it the dark 

 colour of flints was owing, and to its destruction the whiteness 

 of roasted and bleached flints was attributable. 



The lecturer, in conclusion, briefly referred to the formation 

 of some other minerals. He explained that the production of 

 crystals of selenite, celestine, and heavy spar, obviously re- 

 sulted, in many cases, from the sulphuric acid arising one while 

 from burned sulphur in volcanic districts, and at another from 

 oxidizing pyrites, acting upon contiguous masses containing 

 lime, strontian, and baryta. He showed a specimen of red oxide 

 of iron, possessed of a stalactitic form, decisive of aqueous ori- 

 gin ; and oxide of manganese, he smd, sometimes occurred in a 

 similar state. He considered such specimens to have been ori- 

 ginally deposited in the state of carbonates, out of solutions of 

 carbonic acid, and to have been subsequently still farther oxi- 

 dized — a change which he illustrated by a specimen of carbo- 

 nate of manganese, kindly given to him by Mr Philips, in which 

 the progress of conversion was distinctly exhibited. He also 



