STRATIGRAPHICAL ORDER. 21 
_ The Putnam Hill limestone is from 25 to 50 feet above the Upper 
Mercer limestone, and from 50 to 90 feet above the Lower Mercer. 
The more common figures for these intervals would be, respectively, 40 
and 70 feet. 
By the aid of this steady and conspicuous stratum, it is possible to 
follow the horizon of the overlying Ferriferous limestone through the 
counties in which it fails as a regular deposit. The horizon is marked 
by occasional outcrops of limestone or flint, or by calcareous sandstones. 
Seams of ore and coal are also found at the same general level. 
The Putnam Hill limestone does not extend into Pennsylvania, so 
far as is knowr, but its underlying coal can be followed across the bor- 
der, where it becomes one of the well-known and widely extended 
seams of the Lower Measures. It is the Brookville coal of Rogers, 
Coal A of Lesley, with but little doubt. 
The discussion of the Putnam Hill limestone at this time is inci- 
dental to the main purpose, which is to show the continuity of the more 
important Ferriferous limestone. 
Ii]. THe Freeport LIMEsTone (UPPER). 
The Freeport Group is the third in ascending order of the several 
series which extend widely through the Lower Coal Measures, and of 
this group the Upper Freeport Limestone is the most constant and well 
marked element. Like the limestones already described, it was first 
named by the geologists of the First Pennsylvania Survey (vol. II, 
part I, pp. 477, 492, 572 and 579). It is described by Rogers as a 
nodular limestone, in Western Pennsylvania, light blue or dove colored 
when freshly broken, but weathering yellow on account of the iron con- 
tained in it. The nodules are described as imbedded in clay, and as 
containing minute fossils. 
In White’s Report on Beaver county (Second Geol. Survey, Q, 
p. 47), the limestone is characterized as follows: ‘‘ This member of the 
series, unlike its overlying coal, is quite persistent, and retains a some- 
what uniform size and similarity of character over the entire district. 
Jt thus becomes a very important guide. Most generally it is of a light 
gray color on fresh fracture, but it nearly always contains enough iron 
to render it buffish on exposure, and sometimes even enough to con- 
stitute it a valuable ore. . . . It is nearly always more or less 
