STRATIGRAPHICAL ORDER. 87 
counted of exceptionally good quality. This appears to be the seam 
that was struck near the level of Will’s Creek at Morton’s Mills, in 
Cambridge, but a somewhat greater thickness is reported here. Jt may 
be provisionally identified with the Upper Kittanning coal of Penn- 
sylvania. 
The two limestones that are found in the upper portions of the 
last two sections, viz., the Cambridge and the Upper Freeport, serve to 
guide us as we advance to the southward. The Cambridge limestone 
is unmistakable when its stratigraphical place is taken into the account, 
and the same may be said of the Upper Freeport. If either were found 
alone in a section, without other elements to indicate the order, it would 
be possible to mistake the Cambridge sometimes for the Ames, and the 
Upper Freeport for the Lower Freeport limestone, but in such sections 
as are here exposed there is no difficulty in holding the order with the 
same assurance that visible continuity would inspire. 
Both limestones come into the section that is found at Cambridge, 
with many other elements. 
In the hill just west of town, that is pierced by the tunnel of the 
Baltimore and Ohio Railroad, we find a clear and comprehensive section. 
It is represented in Fig. X VIII. 
Four coal seams are found in the Tunnel Hill section. The upper 
one is a persistent bed in the Barren Measures, through several counties 
at least. It is the Norwich coal of Stevenson, and the Anderson coal 
of Andrews. It is frequently found 30 inches thick ,and of fair quality. 
There has been much uncertainty as to which of the three lower coals 
of the tunnel section represents the Cambridge coal, if any one of them 
marks its horizon. In the light of the measurements here recorded, 
and the various elements that are shown in the section, the question 
becomes an easy one, and only one answer is possible. The middle 
seam of the three is certainly the Cambridge coal. It hes 132 feet 
below the Cambridge limestone, it is true, instead of 110 feet, as in the 
last two sections recorded, but all the intervals expand slowly to the 
southward, and every fact agrees perfectly with this explanation. The 
Upper Freeport clay and limestone both appear below the coal in 
thoroughly characteristic form. The Lower Freeport coal is seen at 
the proper interval below, and the Brush Creek coal at the proper 
interval above it. This last-named seam might have been included in 
many of the sections between this point and the Pennsylvania line. 
