HAWorTH.| Detailed Geology of Oil and Gas. 165 
The sandstones generally are of limited extent laterally and 
erade into shales in every direction. Sometimes they grade 
into shales when followed along their outcrop, so that a bed 
which is sandstone at one place is a shale at another. The 
same variation that is found along the strike may be expected 
along the dip, and accordingly shales occurring underground 
and encountered in drilling wells show such variations in 
lithology that no definite sandstone horizons can be differen- 
tiated. 
Wig. 2. Diagram showing gradations of sandstone into shale. 
Figure 2 is an ideal sketch of the relation of sandstone con- 
tained in a shale formation. In this figure the sandstone let- 
tered a is lenticular in shape; it thins out and disappears en- 
tirely from the section, and has no lateral equivalent. The 
sandstone lettered b is shown to grade over into a shale which 
is its lateral equivalent. Sandstone a apparently is the result 
of sedimentation under such conditions that sand only was 
deposited. Sandstone b apparently represents the results of 
sedimentation under such conditions that sands and muds 
were being laid down at the same time, according to the local 
variation in the strength of currents which assorted the ma- 
terial, resulting in a gradual gradation from one into the 
other. As regards these two classes of sandstones, observa- 
tions have shown that the lens-like beds are apt to be more 
persistent and to extend for longer distances, while the sand- 
stones which grade into shales vary from place to place in an 
exceedingly irregular way. 
Drilling operations have shown that in many places in the 
Kansas fields we have sandstone beds occurring under irregu- 
lar conditions which may be explained satisfactorily by giving 
proper attention to the hypothetical cases above outlined. It is 
not at all unusual to find a good, productive sandstone in one 
place and no sandstone at all near by. The Cherokee shales, 
for example, usually are moderately well filled with sand- 
stone of irregular thickness and lateral extent. But some wells 
