STRATIFICATION 115 
pass laterally into fine-grained argillaceous sandstones, and 
these in their turn eventually merge into shales. So lime- 
stone tends to become mixed with clay or sand, and to shade 
off into calcareous shales or sandstones. Again, coal and 
ironstone may mutually replace each other; or each may 
lose its own distinctive character and gradually pass into 
carbonaceous or ferruginous shale. 
An example of a well-marked group of strata which gradually changes 
its character as it extends from one region to another is supplied by 
the Lower Odlite of England. This formation may be followed from 
Somerset through Gloucester and the Midlands to the Humber. 
Throughout its whole course it rests upon and is covered by well-defined 
argillaceous beds—the Lias below and the Oxford Clay above. In the 
south of England the formation is composed essentially of limestones. 
Followed to the north, however, it becomes more and more arenaceous 
and argillaceous, until in Yorkshire the limestones of the southern district 
are entirely replaced and represented by ordinary sandstones and shales 
with associated coals and ironstones. The transformation of the deposits 
is not hard to understand. The calcareous accumulations of the south 
are obviously marine, while the arenaceous and argillaceous deposits of 
the north are of estuarine and brackish water origin. 
A somewhat similar change comes over the great Carboniferous 
Limestone formation when it is followed from England into Scotland. 
In the Mendip Hills the formation consists almost entirely of limestones, 
which reach a thickness of 3500 feet at least. In Northumberland, the 
limestone series of the south is represented by a great succession of 
sandstones and shales, with associated coal-seams and beds of limestone 
that vary individually in thickness from 7 feet to 150 feet—the entire 
formation ranging from 2500 feet to upwards of 6000 feet. In Scotland 
the arenaceous and argillaceous element acquires a very great develop- 
ment—probably not less than 10,000 feet. The only limestones present, 
however, are some half-dozen beds, varying in thickness from a few feet 
up to 20 or 30 yards, which, along with numerous seams of coal and 
ironstone, are intercalated in the upper part of the series. 
Diagonal (Oblique or Cross-) Lamination and Strati- 
fication.—While it is generally true that sedimentary 
deposits are spread out in approximately horizontal sheets, 
- now and again both laminze and bedding show much irregu- 
larity—not only the individual beds, but the layers of which 
they are composed, being often inclined to each other at 
various angles. The structure is shown in Plate XXIV., and 
owes its origin to changes or oscillations in the direction and 
force of currents. Hence it is often termed “ current-bedding ” 
or “false-bedding,” in reference to the fact that the bedding 
