ee ee OO me ¢ ee PF 
—— 
ae 
J. L. Campbell— Geology of Virginia. 445 
not been changed to quartzite. The shales are still nothin 
but fragile shales (with a few exceptions); while the embedde 
limonite iron ores still retain their water of crystallization. 
There has been metamorphism, but only limited, not general, 
except so far as it has been produced through other agencies 
than heat, or even super-heated water under pressure, 
_ 4, Such closed folds as we find in Salling’s Mountain, and 
in many localities among the lower Silurian limestones, seem 
to have been great wrinkles in the strata, pushed upward (or 
downward in the case of synclines), and then pressed together 
by mechanical force acting from a southeasterly direction and 
in a horizontal plane. This is the only way we can plausibly 
account for the numerous troughs and arches and folds found 
along the lines of the several sections we have had under dis- 
cussion. 
ee changes of surface that have given this valley its wonder- 
ul fertility. 
‘here are indications throughout this whole region of two 
great flood periods, since the close of Paleozoic time, when the 
great Appalachian revolution left the vast accumulations of 
Stratified rocks of that remote age in essentially the same rela- 
tive position they now occupy. But further notice of these 
must be postponed for the present. 
