THE CARBONIFEROUS SYSTEM. 83 
ous than to that of the Hamilton. This isshown by the great development 
of the families which include Productus and Productella, Aviculopecten, Pale- 
oneilo, etc., in both the Chemung and Waverly. These constitute a fauna 
which in all its more prominent characteristics is common to both, sery- 
ing at the same time to bring them together, and separate them from the 
underlying Devonian strata. This similarity of fauna is such that it led 
Prof. Hall to unite the Chemung and Waverly long before their strati- 
graphical relations were ascertained, and it has been the cause of much 
of the protracted discussion which has taken place over the age of the Wa- 
verly. I think it may be safely said, that if the question could be disem- 
barrassed of all complications, all intelligent paleontologists would agree 
that the fauna of the Chemung and Waverly belong to one zoological 
age, and that their differences are only such as would naturally charac- 
terize epochal subdivisions of this age. The Catskill group, which nom- 
inally separates the Chemung and Waverly, is a local, and, as yet, very 
ill-defined formation. That it has little representation in the Catskill 
Mountains seems probable, both from the observations reported in our 
former volume and others made since. Doubtless this formation will be 
carefully studied and accurately defined by the newly organized survey 
of Pennsylvania; but with the limited information we now possess in 
regard to it, it seems to me to be a local, and, perhaps, a fresh-water de- 
posit, synchronous with the marine beds of the Upper Chemung. 
A sketch of the history recorded in our Carboniferous strata was given 
in the introductory chapters of the geological portion of our first volume. 
Tt will be remembered by those who have read that sketch, that it was 
shown that a round of physical changes took place in the Carboniferous 
age similar to those traced in the strata deposited in the other great divi- 
sions of geological time, viz.: First—A period of wide-spread land area, 
which in the latter portion of the Devonian age was covered with a lux- 
uriant vegetation similar in its general aspects to that of the Coal Meas- 
ures. Second—At the beginning of the Carboniferous age this land was 
extensively submerged and covered with shore and off-shore mechanical 
sediments, which are now known as the Vespertine and Umbral rocks of 
Pennsylvania, the Waverly sandstone series of Ohio, the Knobstones of 
Kentucky, the Kinderhook group of Illinois, etc. Like other of our me- 
chanical sediments, this group thickens toward the east and north in the 
direction of the land, and thins at the south and west toward the sea. 
Third—In the progress of this submergence, where open water reached 
and stood for ages, caleareous sediment was deposited, which we call the 
Lower Carboniferous, or Sub-Carboniferous, limestone. That this sub- 
mergence in Pennsylvania, Ohio, and Michigan was progressive from the 
