STRATIGRAPHICAL ORDER. 17 
district of Pennsylvania in which it occurs. The distances traversed 
in drill holes before reaching the lower coals or the oil sands are 
generally measured from the Ferriferous Limestone. 
In the region of its best development, the limestone frequently 
reaches and carries a thickness of 15 feet. It sometimes rises to 20 or 
even 25 feet, but over large areas it ranges between 1 and 5 feet in 
thickness. Widely extended though it is, it is still subject to very 
rapid changes in volume, and even to frequent “ wants.” Generally, 
however, the stratum leaves some mark by which its place can be de- 
termined, even though the limestone has entirely disappeared. 
It is the largest and most massive limestone of the Lower Coal 
Measures, and the only one pure enough to be used generally and in the 
large way as flux for iron furnaces. It is charged with a larger and 
more varied series of fossils than any other limestone of the Lower Coal 
Measures. 
In color it is light-gray in its upper portions, and grayish-blue in 
its lower beds, but where the limestone is thin, the whole deposit is often 
of the latter shade. 
It frequently bears a deposit of buhrstone or flint upon its upper 
surface, and this always makes a characteristic and permanent feature 
in the sections that contain it. 
The buhrstone carries the most famous and valuable iron ore of the 
Lower Measures, to which reference has already been made. 
The limestone sometimes exists in two distinct beds, separated by a 
thin stratum of fire-clay or shale. In some instances a small coal seam 
comes between the two beds. When thus separated, there is a marked 
distinction in color between the benches, as noted in a preceding para- 
graph. 
White has called repeated attention to the fact, that whenever in 
Western Pennsylvania or Eastern Ohio the limestone grows thin, it ex- 
hibits ‘cone in cone” structure. (Second Penna. Survey, Q, p. 62, 
Q 2, p. 47, et al). No other horizon near this is known to have this 
peculiar structure, and therefore this mark when recognized becomes of 
practical service to any one engaged in tracing the series. 
Two coal seams occur below the limestone and help to mark the 
general horizon. The place of one of these seams, which is known as 
the Scrub-grass coal, is directly beneath the limestone. The second or 
2 G. 
