Keyes — Fauna at the hase of the Burlington Limestone. 447 



2. The presence of atmospheric air does not restore the 

 sensitiveness, if the silver chloride has been absolutely dehy- 

 drated at 220° C. (Acworth's experiment.) 



3. The presence of oxygen is not necessary or important for 

 the darkening of silver chloride. The presence of moisture is 

 not essential ; its place may be taken by another substance 

 capable of taking up chlorine. This follows from my experi- 

 ment above mentioned. 



There is no doubt that silver chloride retains the last portion 

 of water with great obstinacy. I have frequently tried to dry 

 silver chloride in hot air so that it should lose nothing further 

 by fusion, but never quite succeeded. There is always a loss 

 which may be roughly taken at a half a milligram and from 

 thence upwards to nearly one milligram, in a gram. When 

 the water is thoroughly driven off it is probable that the sil- 

 ver chloride is left in an insensitive condition. Acworth's 

 experiments seem to show this. 



So long as moisture is present the molecule of silver chloride 

 easily breaks up, not merely by the action of light but by the 

 application of any form of energy. The part played by 

 moisture in chlorine reactions is somewhat remarkable. It has 

 been lately stated that absolutely dry chlorine has no action 

 upon copper foil. As soon as a trace of moisture is introduced, 

 energetic action sets in. 



Art. LYIII. — A remarkaole Fauna at the Base of the Bur- 

 lington Limestone in Northeastern Missouri y* by Charles 

 Kollin Keyes. 



In the description of a certain gasteropod (Porcellia nodosa 

 Hall), in the third volume of the Illinois Geological Surveyf 

 occurs the following paragraph, in connection with the 

 assigned horizon and locality : " Lower Carboniferous ; Barry, 

 Pike county, Illinois; from a peculiar cherty, calcareous band 

 at the base of the Burlington limestone, formerly supposed to 

 belong to that rock, but now known to contain fossils charac- 

 terizing the oolitic upper bed of the Kinderhook group, at 

 Burlington, Iowa." 



This allusion though merely incidental is the only direct 

 one ever known to be made to the particular beds now under 

 consideration. 



* Published by permission of Mr. Arthur "Winslow, Director of the Geological 

 Survey of Missouri, from work prosecuted during the years 1891-92. 

 fGeol. Surv. Illinois, vol. iii, p. 459. (1868). 



Am. Jour. Sci. — Third Series, Vol. XLIV, No. 264. — December, 1892. 

 BO 



