1887] The Materials of the Appalachians. 961 
_ phenomena., Let but the sea-margin, which was the loaded area, 
_ sink somewhat faster than the dry land, which was the lightened 
_ area,and we have all the conditions that were needed for the 
erosion and denudation of the relatively rising surface to any 
_ fequired amount. The only limit is some physical change which 
_ could put a stop to the rising of the one or the sinking of the 
_ other, and this came at the end of palzozoic time, when the 
_ Appalachian trough became full. 
_ Another conclusion may be drawn from the facts stated in the 
_ Second part of this paper. I have pointed out the enormous 
_ quantity of white vitreous quartz-pebbles that lie in the palzozoic 
= sandstones of Pennsylvania, and have shown that their only 
_ Possible known source is the quartz-ledges of the South Moun- 
tains, I have also shown that this source is very small when we 
_ Consider the total mass of the pebbles. It follows, therefore, that 
at the time when these pebbles were made—that is, during the 
_ Paleozoic era—there must have been a larger supply of white 
_ Quartz to afford their material. In other words, the pre-paleozoic 
_ Tocks, with their quartz-ledges, must have been much more ex- 
tensive then than they are now. The narrow strip along the 
South Mountains and their continuation in Pennsylvania, New 
Jersey, and Maryland, so far as the present subject is concerned, 
; Can scarcely exceed one thousand square miles in area. It is 
hard to avoid the inference that vast masses of ancient strata 
have totally disappeared from this region. Possibly such masses 
isted, overlying more or less of the archæan area in the 
“Southeast. If so we have at the same time a sufficient quarry 
from which the rains and waves of that age could obtain their 
_ Material and a body of rock hard and thick enough to keep 
E Mem employed during those millions of years. 
y attempt to discuss further the place and extent of this 
_ i@Surface must be reserved for another time. It may have 
: verlain more or less of the soft gneiss-rocks of Pennsylvania and 
: ~~ been since removed. It may have been crumpled into ant 
: inal folds with the gneiss, as are now the palæozoic rocks; or it 
Pad to a greater or less extent have been contracted in ae by 
7 ben endous lateral compressing force to which this region has 
‘Subjected. Or, again 
x or Parts of the State and in other places in the world. All 
