STRATIGRAPHY. 19 
it occupies a strip over a mile wide on the hillsides southeast of and 
parallel with the creek. In the valley of Little Chartiers Creek it 
extends from the edge of the quadrangle as far south as Wylandville 
and outcrops up the side valleys for 2 or 3 miles. In the northeast 
corner of the quadrangle nearly all of the area north of the Williams- 
port pike and east of a line drawn northward along Snipe Run, with 
the exception of a few hilltops, is covered by rocks of the Mononga- 
hela formation. A small area along Peters Creek consists of Cone- 
maugh and Quaternary deposits. The Monongahela formation out- 
crops on the several branches of Pigeon Creek as far as Emery and 
Vance ville, and nearly to Three and Four. It also forms the greater 
portion of the area southeast of ZoJlarsville and Spring Hill. 
ROCKS WHICH DO NOT OUTCROP. 
GENERAL STATEMENT. 
Sources of knowledge. — Information concerning the rocks which do 
not outcrop is derived entirely from the records of deep wells bored 
for gas and oil, and is, therefore, somewhat imperfect. In many cases 
records have been carelessly kept and beds important from a geologic 
standpoint, such as coals, bands of red rock, or limestones, have been 
overlooked or not recorded. In many cases only the oil and gas sands 
have been noted, thus leaving great gaps in the records. The meth- 
ods of measurement also introduce some errors. While measure- 
ments to the principal oil and gas sands are frequently made by steel 
line, and are accurate, the depths and thicknesses of the various beds 
are generally determined by counting the turns of the cable on the 
shaft of the bull wheel, and errors may easily occur. In deep wells 
the stretching of the cable may cause an error of considerable magni- 
tude. The difficulty of identifying rocks by the relative ease with 
which the drill penetrates them and by the drillings brought up in the 
sand pump is also likely to be a source of error. To this cause may 
be due many of the lithologic variations recorded in well sections. It 
may thus happen that important beds which are not recorded are not 
really absent, but have been overlooked. In some cases a heavy 
sandstone in one well might change to a highly arenaceous shale or 
shaly sandstone in a near-by well, and thus be regarded as " slate" or 
shale. At best, observations on rocks in deep- well sections must be 
confined almost wholly to their lithologic character. It is usually 
impossible to learn anything of the fossils by which the ages of the 
rocks might be determined. 
Thickness. — The greatest thickness of the rocks pierced by the 
drill in the Amity quadrangle is in the Mrs. A. L. Hawkins No. 1 well 
(31 rt ), 1.2 miles southwest of Beallsville. The total depth of this 
a Numbers in parentheses refer to locations of wells on the map (PL I, in pocket) and records given in 
the table, pp. 70-87. 
