1885. ] of the Appalachian Mountains. 259 
From this list I shall exclude the first because it consists in 
great part of gneissic rocks whose bedding is doubtful and 
difficult of detection, and the thirteenth because its strata are of 
later date than those of the other ranges. There remain, there- 
fore, for discussion eleven distinct mountains, rudely parallel, and, 
for Pennsylvania, of the first order. To prevent misconception 
or objection I should state that all these ranges are composed, 
medially, of the same rock—the massive Medina sandstone—the 
base of the Silurian system proper in Pennsylvania. The possi- 
ble error of counting the same dip more than once is, by this 
fact, evaded. 
The line of section above mentioned is sixty-five miles long, 
and may be divided naturally into two parts. One extends from 
its north-western end to the Blue mountains, and the other from 
the Blue mountains to its south-eastern end. The first division 
is forty-nine miles long and crosses all the eleven ranges of moun- 
tains above named. The second is sixteen miles long, and 
crosses only the Cumberland valley. 
The line lies in six counties, among which it is distributed as 
follows : 
Blair 2 miles. 
Huntingdon 24 «8 
Mifflin oS 
Juniata poom 
erry 9o 
Cumberland........ ap ee 
65 
The accompanying plan exhibits the relation of the eleven 
ranges to one another, and the section shews the position of the 
bed of Medina sandstone which forms the axis of each. 
The problem is now reduced to finding the length of the line 
representing this bed of sandstone in its present contorted condi- 
tion, for that must represent, approximately at least, its original 
extent when spread out flat. 
Taking first its north-western portion forty-nine miles in length 
I have drawn it as nearly as possible to scale on the accompany- 
ing diagram. I propose now to make a deduction of twenty 
miles for the flattish tops of the arches and bottoms of the 
troughs—an allowance which is, I think, in excess of the truth. 
This deduction leaves twenty-nine miles of strata, dipping more 
or less steeply, often standing nearly vertical, sometimes over- 
