J. J. Stevenson— Upper Devonian Rocks of Pennsylvania, 425 
serve unity in the maps of the survey, is not in full accord 
with the facts. 
To determine the relations of rocks one may be guided by 
lithological characters and relative position, or if possible he 
may trace the rocks to some typical locality, or should fossils 
be present he may make his determinations by means of those. 
For the most part, geologists are satisfied to abide by the last 
test, as itis of universal application and saves a great expendi- 
ture of time and labor. But some geologists are disposed to 
think the simpler method inaccurate, and seem inclined to rebel 
against an imagined assumption on the part of paleontologists. 
It is desirable then to ascertain whether or not the relations of 
these rocks can be determined by tracing or by lithological 
characters. 
The bold anticlinal axes of southwest Pennsylvania are the 
Alleghany Mountains, Negro Mountain, the Viaduct axis, 
Laurel Ridge and Chestnut Ridge, all mountainous for the 
greater portion of their extent within the State of Pennsyl- 
vania. Under these axes alone may one look for exposures of 
the lower strata, for away from them the surface rocks belong 
to the Coal Measures. 
An exposure under the Alleghanies in Maryland reaches 
below the Pocono or Vespertine sandstone, but northward 
there is no described exposure anywhere on the west side of 
those mountains in Somerset County of Pennsylvania;.and, as 
be ascertained from the report of Mr. Platt’s close 
survey, the deepest gorge on that side is cut down only to the 
rocks of Formation XI, the Umbral. But in Cambria County, 
which is immediately north from Somerset, the exposures 
The Viaduct axis separates itself from the Negro Mountain 
in northern Maryland and continues as a strong axis through 
Somerset, Cambria and Clearfield Counties of Pennsylvania. 
But it nowhere shows anything below the upper portion of 
Formation X, as appears from the reports made by Messrs. F. 
and W. G. Platt. 
Laurel Ridge, at the line between Pennsylvania and West 
Virginia, niente only the upper portion of the Umbral, XI, 
but at the Youghiogheny River, the upper part of the Devonian 
is reached, its section being ex by the railroad cuts. 
Here and there, northward from the Youghiogheny, a deep 
