Concretions in Southern \c:v )'ork .—Kindle . 363 
5Ftr- 
-l3Ft-r- 
FiG. 3. Showing rcllcxed fossiliferous zone. 
on the sediments before their complete consohdation, since 
there is no evidence of fracture or crushing in the l^eds nor of 
distortion in the fossils. If it he conceded that the reflexed 
condition of the fossil band was brought about soon after the 
deposition of the associated sediments, while they were in a 
semi-plastic condition, the development and expansion of gases 
beneath an impervious zone would account for the disturbed 
condition of the zone of fossils. The tendency of a gas accu- 
mulating beneath impervious beds would be to lift them up- 
wards along lines of least resistance into approximately vertical 
positions, such as we find the fossil band to have assumed. It 
appears not improbable that the gases supposed to have dis- 
turbed the normal position of the strata associated with the con- 
cretions may have been a factor in inciting the chemical segre- 
gation which has developed a composition unlike that of the 
associated beds in this and other bands of concretions. 
The behavior of gases in recent mud in a manner somewhat 
scimilar to that which is here assumed has been observed by 
Aggasiz and Hosford. The latter states that they observed 
the "production of raised hemispherical surfaces in the mud 
near Cambridge by the rising of gases from decomposing or- 
ganic bodies.''* 
• Am. Assoc. Adv. Sci.. vol. iv, p. 12. 
