The Making of Pennsylvania. — Claypole. 227 
main features are traceable. He cannot tell how large was the 
mass of land whose outlines formed the coast whether it was con- 
tinuous or archipelagic, high or low. Some have thought that 
they could make out a vast continent extending into what is 
now the Atlantic ocean. Others fail to see more than a narrow 
strip of land ending eastward nearly at the existing coast. Some 
picture high mountain ranges that have been since destroyed 
by erosion. Others see only a low tract more or less cut up 
into islands of gneiss and schist and slate. 
What we do know is that the bottom of this sea of Pennsyl- 
vania above mentioned was slowly subsiding during the whole 
palaeozoic era, not continuously but intermittently, as the 
wreckage washed off the adjoining land was dropped into it by 
rivers and currents. Such subsidence under increasing load 
is a common occurrence. It is now happening at the mouths 
of the Ganges and the Mississippi. Its frequency has supplied 
a strong argument to those who favor the theory of a yielding 
crust underlaid by a liquid or viscous stratum. But be this as 
it may the fact remains that a region of great deposition is 
usually a region of coiitinued subsidence and if in the present 
case the proof be demanded it is readily forthcoming. In the 
immense mass of sediment laid down in the palaeozoic sea of 
Pennsylvania we find indications of shallow water in ripple 
marks, mud-cracks &c.,and as these occur at almost every level 
through the whole five or six miles of rock that compose the 
deposit it is evident that every such layer was at the time of 
its formation within reach of surface action. 
Probability not less than evidence indicates that then, as 
occurs now, the marginal coast-line rose as the sea-bottom fell 
and that thus there was a continuous renewal of the quarry 
from which material could be cut by erosion and of the pit 
into which it could be thrown. 
We may then picture to ourselves all the northern, central 
and western parts of the state in the early palaeozoic period 
under the waters of a slowly deepening sea and the southeast- 
ern corner as a slowly rising area of land. Beyond this we 
dare not go with confidence. What rivers washed the worn 
material of that land into that extinct sea, where they flowed, 
how large they were and what changes they underwent during 
their long existence — all these matters are loft for the future to 
disclose. What rocks then constituted its surface Ave can do 
