MISSISSIPPIAN SERIES. 27 
that direction. In the Gantz well, for instance, the interval from the 
Pittsburg coal to the top of the Big Injun sand is only 1,111 feet. 
On the assumption that 600 feet is the most probable thickness of 
the Conemaugh, 300 feet of the Allegheny, and 150 feet of the Potts- 
ville, there is left only 61 feet for the Mauch Chunk. According to 
correlations in PI. IV (p. 44), the thickness in this well may amount 
to as much as 107 feet, consisting of 77 feet of black shale and 30 feet 
of limestone; but even this thickness is much less than that of the 
formation in other parts of the quadrangle. 
Unconformity. — The irregularity in thickness of this formation is 
prominent throughout western Pennsylvania, and is caused by an 
unconformity between the Mauch Chunk and Pottsville formations, 
due to erosion after the deposition of the Mauch Chunk, and preced- 
ing that of the Pottsville. Owing to this feature the Mauch Chunk 
dies out entirely toward the north and west. The thinning is illus- 
trated by many of the well sections in the Burgettstown quadrangle, 
northwest of the Amity. For instance, in the McKnight No. 3 well, 
in Chartiers Township, the interval from the top of the Salt sand to 
the top of the Big Injun is only about 190 feet. In some wells in 
Smith Township it is as low as 100 feet. Nowhere in this area is 
red shale reported in the interval. In two Caltergahn wells in 
Chartiers Township 35 and 103 feet of black shale are reported. In 
certain wells in the Burgettstown quadrangle this shale is entirely 
missing, and the Salt sand rests directly upon the Big Injun. One 
such occurrence has been reported in the northern part of the Amity 
quadrangle, in the Thomas Templeton No. 1 well (111), at Linden. 
Just where the Mauch Chunk formation disappears, whether the 
black shale between the Salt and Big Injun sands is Mauch Chunk 
or Pottsville, and whether all reports of the running together of the 
sands are correct, are questions which at this date can not be defi- 
nitely answered. 
Greenbrier limestone {Big lime). — This bed is well developed be- 
neath the greater portion of the quadrangle, but northwest of Wash- 
ington it seems to die out. A thinning toward the north has also 
been noticed by Campbell in the Latrobe and other quadrangles. 
The disappearance of the Greenbrier toward the northwest is entirely 
independent of the unconformity mentioned above. 
From the well sections it will be seen that the limestone is frequently 
double, and this feature becomes more conspicuous toward the south- 
west. The upper bed is known as the Little lime and the lower bed 
as the Big lime, and they are usually separated by a thin, soft shale, 
which breaks up in a very peculiar manner into small pieces about 
the size of a slate pencil. The drill sinks rapidly in this shale as it 
emerges from the hard limestone above, and the shale tends to "cave 
in" after removal of the drill. The parting is therefore known to 
the drillers as the Pencil cave. 
