Stratification Pla?ies. — Keyes. 297 
separating the terranes, formations or geological units, and in- 
dicated in mapping or selected for special recognition, are 
usually readily made out on account of a more or less abrupt 
change in the lithological characters of the contiguous rock 
masses. 
These formational planes may be regarded as conforming 
to huge warped surfaces. They follow the upper or lower faces 
of the irregularly lenticular rock masses constituting the geo- 
logical units. In cross-section, they form broadly curved lines. 
They are significant of very marked changes in the physical 
conditions of the region at the time the deposits were laid 
down. These changes may have been produced by diverse 
agencies, the exact nature of which is, in the present connec- 
tion, immaterial. These planes approach and emerge into one 
another, regardless of any observable law, though there is no 
doubt some close interdependence between them and the con- 
temporaneous drainage lines of the neighboring shores. 
Seen usually only in cross-section, as a mere line between 
terranes, the formational plane has heretofore received no 
more attention than the ordinary local bedding plane. Since, 
however, of recent years the "formation" or terrane has come 
to be recognized as the primary geological unit for all classi- 
fication of rock-masses, and for mapping, the planes bounding 
these units assume an importance not heretofore possible to as- 
sign to them. They no longer should be looked upon in the 
same light as simply bedding planes. 
Great Planes of Sedimentation are those that separate rock 
series in the same geological province. They may coincide lo- 
cally with formational planes and even bedding planes. They 
may be well defined in one part of a province and in another 
part pass through the very middle of a rock succession that is 
apparently homogeneous and unbroken in all lithological, fau- 
nal and other characteristics. The real nature of these planes 
usually comes out only after the geology of the region has been 
carefully worked out. 
A concrete example, in a limited province, is a coal horizon, 
whether yielding mineral fuel or not. Also, a limestone, that 
in the direction of an ancient shore passes insensibly into shale 
and then into sandstone, may have one of its faces on the hori- 
zon marking the line of division between two great rock series. 
