'^i't^^ y/lr AllirrlcUll (iil>J<Kllst . J)o<cnil)('r, IHltl 
of I'i'rric oxide. Vet in .such cii'dimstiinci's tlic liim'sloiu' has 
<;iven way 1o the dissolving action of passing waters, Icaohcd 
insoluble rcsithics retaining its original structure, as well as 
moulds of fossils, occupying its place along with creepings from 
adjacent strata. Here transmission of seepage water has proved 
too rapid for other liiaii solvt-iit or destructive action to have l)e- 
come sensible. 
Another local circumstance is also deserving of mention. It 
is this. Opposed to a rapid transmission of infiltrations in water- 
sheds between successive cross- ravines is a harrier known as Red 
liidge. Constituted of a compact series of arenaceous argillite, 
this is locally developed at the top of the Surgent shales, and 
stratigraphically above, but topographically in front of, the plane 
<)i the ore-limestone. Near where this barrier is scored by cross- 
ravines, underground as well as superficial drainage has become 
accelerated. ['reservation of Ked Ridge from local erosion has 
therefore come to l)e regarded in Hedford and Huntingdon coun- 
ties, Pennsylvania, as indispensable to a favorable development 
of the fossil-ore bed back of it. or to the absence of • -wants. '" 
Of these the distribution and extent are thus mainly determined 
by conditions of underground drainage as att'ected mostly' by 
topographical features, (rentle dips under steep slopes are for 
obvious reasons inconducive to infiltration. 
The al)Ove remarks directly apply to the more or less hydrous 
fossil-ores of the Appalachian ridges in southern Pennsylvania, 
as distinguished from oolitic hematites, or dyestone ores, like- 
wise developed in favorable ciicumstances on lower horizons of 
thin crinoidal limestones within compass of the Clinton or Sur- 
gent formations. Both tyjx's of ores, and often both series of 
developments, are generally referred to inditferently as Clinton 
fossil-ores. The stratigraphical relations between these two series 
of developments, even where both may be recognized in a single 
ridge or section, are extremely varial)le. In southern Pennsyl- 
vania, where the Clinton shales attain a thickness of nearly 1.2(10 
feet, the fossil-ore l)ed is about 400 feet above the horizon of the 
Frankstown oolitic or dyestone ore. which in tinn is about ;>00 
feet above that of the block-ore, so-called. .\11 of these ores owe 
their development, as T l»elieve, exclusively to secular replace- 
ment of elevated parts of these limestones — not. as sometimes 
explained, to direct sedimentation in whole oi- in part. For 
