The (J'uicinnati Rock.s. — Pcrri/. 835 
through transportation to quieter parts, they were allowed to 
settle. The mineral constituents of the decayed plants had a 
somewhat similar history, though being in an almost inpalpa- 
ble form would remain in suspension longer. If this were 
true and all that occurred, we should find the lower portion 
of each stratum of stone nearly pure carbonates of lime and 
magnesia, but gradually merging into a nearly pure shale at 
the top. But we have assumed a shallow sea, whose bottom 
wa^ scarcely ever free from the jigging action of surface 
waves. This action would gradually sort the constituents of 
the last-deposited stratum according to size and specific 
gravity. This stratum would eventually be sharply divided 
into two — according to the character of its constituents — the 
bottom layer being composed of mechanically triturated frag- 
ments of shells. 
This theory does not imply a limited or definite time for 
the formation of the two strata, for after being once deposited 
they may both have been many times taken into suspension 
partially, at least, by more violent disturbances of the water, 
and again deposited. Each repetition of this redeposition 
would tend to mark more sharply the line between the two. 
Finely ground limestone suspended in water tends upon set- 
tling to become quite compact. The writer has experimented 
on this and has obtained a firmer mass by the mere action of 
gravity than he could obtain by compressing the same mate- 
rial by hydraulic power under a pressure of five tons to the 
square inch. 
We may suppose that the purer limestone, separated out as 
above, may have formed a stratum unacted upon by the gen- 
tler movements of the water, while the overlying shale was 
still being kept in motion. In this way the limestone stratum 
might continue to increase in thickness for long periods of 
time, retaining its great purity and always leaving a well 
defined surface upon which the overlying shale might be com- 
pacted when a favoral)le opportunity, long enough continued, 
should arise. 
This shale, should it be allowed to settle thoroughly and 
remain sufficiently long in this condition, might become 
coherent enough to resist the further jigging action of the 
water. A new deposition would now commence and after 
passing through the same stages we should have two more 
strata — one of limestone and the other of shale. 
