Mauch Chunk of ‘Pennsylvania.—Stevenson. 243 
the eastern border of Armstrong county. This portion is per- 
sistent southward as a sandy limestone of peculiar structure, 
termed the Silicious Limestone in the writer's Pennsylvania re- 
ports. The upper or great mass of the limestone, however, 
though faintly recognizable in central and northern Pennsyl- 
vania, does not become noteworthy until the southwest coun- 
ties of the state are reached, under the bold axes of the Alle- 
gheny plateau. The most northerly point, at which it has been 
recognized definitively, is in the Conemaugh river gap through 
Chestnut ridge in Westmoreland county where six feet of 
fossiliferous limestone and three feet of calcareous shale, sep- 
arated. by sixty feet of shale and sandstone, are shown in the 
railroad cuts. These are represented, probably, by I. C. 
White’s calcareous breccias on the west side of the Broad Top 
field. Southward from the Conemaugh, these upper limestones 
thicken rapidly and attain much economic importance within 
twenty-five miles, exposures being frequent within Fayette 
and Westmoreland counties in the gaps through Laurel and 
Chestnut hills as well as several localities further east in Som- 
erset county. These limestones, so insignificant at the north, 
become the notable portion in the Virginias. 
The upper shales persist in northern Pennsylvania into 
Warren county, but they disappear westward so that they are 
indefinite throughout the extreme western counties of the 
state; within the central portion of the basin, however, they 
retain their characteristics to southern Virginia. 
The United States Geological Survey has divided the 
Mauch Chunk or Umbral. The upper shales are termed the 
Mauch Chunk, the limestone is the Greenbrier, but no desig- 
nation has been affixed to the lower shales, as they have not 
been encountered sharply within any quadrangle yet mapped 
by that survey. It is unfortunate that these terms have been 
adopted, since they are as objectionable—and for the same rea- 
sons—as Chemung and Hamilton, both of which have been re- 
jected by the survey for lack of definiteness. Mauch Chunk 
was applied in Pennsylvania to the whole series above the Po- 
cono and includes the Greenbrier as well as the underlying 
shales: the writer in 1878 suggested the name Greenbrier for 
the whole series above Pocono and W. B. Rogers in 1883 used 
the terms Greenbrier shales and Greenbrier limestone. It is un- 
