960 The Materials of the Appalachians, 
this material in the South Mountains, where the pre-palzozie 
rocks rise to-day from beneath the overlying strata. : 
Here, and here only, in Southeastern Pennsylvania can be 
found the white vitreous quartz of which they are uniformly 
composed. No other rock can have supplied them. Theres 
abundance of quartz in the gneisses near Philadelphia, and in 
Chester and other counties, but this quartz is not of the kini 
from which these pebbles are made, nor does it occur in masss 
sufficiently large to supply them. Some of the other materials 
of the palæozoic sandstones may be ascribed to the destruction 
of the gneiss-rocks. For the white pebbles, however, this origit 
is not possible. 
But though I have thus tracked these pebbles of the Pennsyi- 
vanian Conglomerates to their only possible source in the soute 
east, a serious difficulty stands in the way of further progress. l 
have, without giving figures, dwelt on their enormous quantity 
and the vastness of the mass of quartz which they represent. 
Now, the quartz-ledges of the old rocks in the South Mountains 
are not thick, measuring only a few feet in most cases. Norat 
they numerous, They could never furnish quartz enough r 
supply even the existing pebbles; and the extinct pebbles— 
those ground down to sand—must have been even more thi 
those now surviving. If we break up all the quartz-ledges * 
present existing in the South Mountains, and eke them out N” 
the quartz-veins penetrating like threads the mass of the other 
strata, these, when rounded into pebbles, will sink into utter 
_ Significance beside the mountain which would rise were all ; 
remaining quartz-pebbles of the Pennsylvanian Conglomerat 
collected into one place and piled together. - pa, 
-In conclusion, then, it follows from the facts here put fo i 
whether the figures given be exact or not, that an al # 
credible amount of material was eroded from the archeat call 
of North America during palæozoic days. In no other ra 
we account for the enormous palzozoic deposits of the e 
and Midland States. Yet it is not probable that the lané © 
stood at any dizzy height above the sea. The slow 
pression of which we have so abundant evidence from 
of the globe, and which in this very region has 
Pennsylvania down at least thirty thousand or forty: > 
feet below its former level, is quite sufficient to account 
Middle 
for ti 
