GENERAL GEOLOGY OE CLAYSVILLE QUADRANGLE. 
51 
to more than 4,000 feet. Drillers have noted and named certain 
prominent or easily identified beds found in the Avells, and have in 
most instances recorded the depths at which each of these beds was 
encountered. Owing to the uniform and widespread occurrence of 
the Pittsburg coal and the ease with whjch it is recognized by the 
driller, it has come to be universally used in this region as the base 
from which the intervals to underlying beds are measured. This 
interval and the known sequence of the beds form the principal 
means by which they are identified, though some strata have indi¬ 
vidual characteristics by which they may be recognized over large 
areas. In this connection it should be stated, however, that there 
are certain other factors to be taken into consideration and it is 
necessary to accept information from well records with caution, 
^ome drillers are careful and exact both as to measurements and 
the correlation of sands; others are careless and neglectful, giving 
little heed to any rocks except those producing oil or gas. In a 
few instances where the information is conflicting in a given area 
it has been difficult, for this reason, to choose between the good and 
the bad. Another source of error is in the measurements themselves. 
Especially is this true where measurements have been taken by cable 
to unproductive beds and by steel line to the productive sands in 
the same well. Where a steel line is used the distance is measured 
very accurately, but with the cable as a measuring line the results 
are as a rule unreliable. (For a detailed description of these 
methods, see p. 29.) 
SECTIONS OF SUBSURFACE ROCKS. 
In order to illustrate the relative position and thickness of the 
prominent subsurface beds within this quadrangle the records of six 
wells, situated on a zigzag line across the quadrangle from south to 
north, have been selected to show typical sections of the rocks of the 
vicinity from which each is taken. It is believed that a careful 
examination of these sections (PL XI) will give a clearer and more 
comprehensive knowledge of the relative thickness and stratigraphic 
relations of these beds than could be conveyed by pages of description. 
BeloAV the Pittsburg coal are several prominent sandstone beds of 
the Conemaugh, Allegheny, and Pottsville formations down to the 
unconformity betAveen the Pottsville and the Mauch Chunk, the 
Avhole being generally designated the shallow sands. The second 
group or upper sands extends from the top of the Mauch Chunk 
through the Pocono formation, the bottom of Avhich is probably a 
short distance above the Gordon Stray sand. All sandstone beds 
below this horizon in the upper portion of the DeAmnian are gen¬ 
erally designated the lower sands by the oil fraternity. 
