J, LeConte—Oritical Periods in the History of the Earth. 99 
the sensitiveness of the silver haloids to light, do so, not by 
forming substitution compounds with the halogen, but by pro- 
moting, in virtue of their affinity for oxygen, the decomposition 
_ of water by the halogen. : 
There is, probably, another affinity which plays some part in 
the tendency to form the halogen acid, and that is the tendency 
shown by the silver haloid to form combinations with the cor- 
responding halogen acids, e. g. hydroargentic iodide AgIHI, 
etc., in the absence of an alkali. 
Philadelphia, June 26, 1877. 
Arr. XVI.— On Critical Periods in the History of the Earth and 
their Relation to Evolution: and on the Quaternary as such a 
Period ;* by JosepH LeConrn. 
of the unconformity. These general unconformities attended 
boundaries of the great divisions of time, and the less genera 
sai ly; and that these exterminations were followed b 
peereations of other and wholly different species at the begin- 
oe & of the subsequent period of tranquility. Now, however, 
"e believe that no such instantaneous general exterminations 
forms oreations ever occurred. Now we know that uncon- 
amity simply indicates eroded land-surface, and therefore 
ae 4 period of time during which the observed place was 
nd and received no sediment—that two series of rocks uncon- 
ae le to each other indicate two periods of comparative quiet, 
“ring which the observed place was sea-bottom receiving sedi- 
ily, separated by a period of. oscillation at pains 
u 
* Read before the National Academy of Sciences, April 18, 1877. 
