1014 General Notes. 
Now, however applicable these statements may be to the Devon- 
ian rocks of other States, they certainly are not, to any such degree, 
applicable to the rocks of this age in Iowa. The Devonian rocks 
of this State, as observed by Dr. White,! are serially isolated. 
The occurrence of extensive beds of very coarse and fine-grained 
sandstone, varying from five feet to forty feet in thickness (some- 
times obliquely and discordently stratitied throughout), and blue 
and buff shales, some of which are extensively sun-cracked, and 
others containing abundant remains of land plants, as well as exten- 
sive beds of blue clay and hard, fine-grained and compact lime- 
stone, and the varying fauna of the several divisions, all attest that 
there were nearly equally as striking alternation of conditions during 
the successive deposition of strata in Iowa as are indicated “ at the 
east;” and that the alternating shore and off-shore conditions which 
interposed sheets of chemical and mechanical sediment, have, to a 
greater or less degree, given a distinctness to the fauna of the several 
divisions equal to that “ of the east.” ; 
An interestitfg and instructive feature of the exceedingly rich 
fauna of the Rockford Shales of Iowa is the extreme minuteness 
of a considerable number of its fossil species. Of these minutissi- 
mic, yet adult, forms, more than fifteen are Gastropoda, four 
Brachiopoda, three Crustaceans, four or five Foraminifers (suborder 
Perforata), and five or six small Bryozoan corals. These forms, 
which comprise slightly over one-seventh of all those known to 
occur in this formation, are usually well represented both as to 
genera, species and individuals. ‘ 
The organic life of the old Devonian sea in Iowa culminated in 
these shales; and at this time was ushered in a period when the 
conditions were much more favorable to the existence of life than 
at any other epoch of the Devonian age in this State. Immediately 
underlying the Rockford Shales is a stratum of dark blue be! 
from twenty. feet to twenty-five feet in thickness, and se 7 
destitute of organic remains; this attesting conditions, when r 
material was laid down, extremely unfavorable to the existence 0 
any form of life. Upon the ushering in of conditions under ego 
the material of the super-incumbent shales was deposited, t 
change from the pre-existing conditions was very abrupt.’ 
Although the change in the character of deposition here ie 
very sudden, yet the change as to congeniality to life seems no | 
ve been so rapid, as appears to be attested by the fact that near M 
all the depauperate forms above enumerated occur at the base 
these shales. 
1 r ? 7 
2 Sahel gies Bi beds or peated from the blue clay below to "a 
id and 
shales above, the change having been everywhere very rapid aD 
sharply defined. 
