230 The American Geologist. April, mo 
and Chemung. The indications of slight or temporary changes 
of level during this long succession, though of much geological 
interest, must not detain us here. But the immense thickness 
of these rocks combined with the fineness of their material 
enables us in some degree to realize the enormous lapse of 
time required for their deposition. 
A change then ensued in the nature of the sediment. The 
deposit of fine mud was succeeded by the inbringing of sand, 
and the Catskill or Upper Devonian strata were laid down to 
the thickness of 8,000 to 10,000 feet. There are strong reasons 
for believing that these red rocks were formed not in a marine 
but in a lacustrine or aestuarine area and that some geograph- 
ica change had taken place which is as yet unknown but 
which had the effect of cutting off that free communication 
with the open sea that had previously existed. Then followed 
the Pocono sandstone 2,000 feet in thickness, the Mauch 
Chunk red shale 1,500 feet, and the Carboniferous Conglomer- 
ate of about 1,000 feet on the top of which the coal beds were 
formed in succession consisting of alternating strata of shale 
and coal with few limestones and indicating fresh water or 
marshy conditions during a great part of the time. 
The long Appalachian subsidence was still continuing, but 
apparently the supply of sediment from some new source 
enabled deposition to keep pace with it and during the Carbon- 
iferous era we find evidence of alternating conditions. At 
one time the sea which had survived in the southwest, in 
Indiana, in Illinois, Missouri and Tennessee, extended into the 
southern and western counties of Pennsylvania, depositing 
marine limestones and marine shells. Then fresh water 
resumed the area and even dry land supported a rich and 
varied vegetation. Anon these were buried beneath a deluge 
of sand or mud. Again the ground became dry or swampy 
and coal plants flourished. Subsidence prevailing brought in 
the sea from the southwest and a limestone was the result. 
Thus the process continued until a heterogeneous mass of 
strata was deposited ranging from 10,000 to 14,000 feet in 
thickness. The picture of the Keystone state during this time 
is one that requires much thought Jlnd trouble to realize. The 
constant and long succession of changes implies not only a 
long time but a condition of instability now unknown. The 
flow of streams or seas of apparently fresh water bearing 
