4 1887] The Materials of the Appalachians. 959 
_ the name of Conglomerates. The pebbles vary in size from that 
= ofa man’s fist or larger in the east to that of a hazel-nut or a 
E pea in the west; but they extend over the whole area, and form 
= aconsiderable part of the mass of the rock. They are as con- 
_ Spicuous in Ohio as in Pennsylvania, and everywhere closely 
_ tesemble one another. 
It would be impossible to form any just estimate of the quan- 
_ tity of these pebbles in the mass of the sandstone. In the first 
_ place, their occurrence is uncertain. Over large areas much of 
the rock is free from them, except in certain, or rather in un- 
= certain, layers; and, secondly, almost everywhere except in the 
4 fast their proportion, even in the conglomeratic beds, is very 
Variable. Not unfrequently they form seventy-five per cent. of 
q the stone; at other times only an occasional pebble is present. 
But, scattered as they are over so great an area and through so 
7 great a thickness of strata, their whole quantity must be enor- 
_ Mous. Were we to assume that they compose only one per 
“Gat. of the rock they would equal a bed thirty feet deep, with 
40 area of thirty-six thousand square miles. 
= Moreover, their rounded form, without a remaining angle*in 
_ aly case, proves two points,—first, that the surviving pebbles 
have been greatly rubbed down from their original size, es- 
= sve not survived at all, but have been ground into sand by the 
_ Wear and tear of their long pilgrimage over the ocean-bottom or 
Tn whatever way we regard them, and from whatever point of 
, ‘ial no doubt can be entertained that these pebbles of our 
Pennsylvania Conglomerates represent, in the whole, a huge 
‘Mass of milky quartz, and that this, in like manner, represents a 
__ 8*t Mass of the same material, from which, by crushing and 
ig, they have been derived. Every one is a monument, so 
— to the memory of an angular block, broken, at some 
: “distant day, from its parent-ledge, and rolled for years among 
Som and sand until it was reduced to a smooth, small pebble, 
buried to rest in the Conglomerate Rock. 
= S Was this mass of milky quartz, the parent of the un- 
d pebbles of the Conglomerates of Pennsylvania? Milky 
artz is not a common mineral in large quantities. I know no 
to which they can be ascribed except certain ledges of 
pecially in the west, and, second, that innumerable fragments 
è 
