

Part 1] PERSISTENCE OF STRATA. 493 
by a considerable thickness of intervening sandstones and shales, 
ean be traced through the coal-fields of central Scotland over an 
area of at least 1000 square miles. Coal-seams also possess great 
persistence. The same seams, varying slightly in thickness and 
quality, may often be traced throughout the whole of an extensive 
ceoal-field. 
What is thus true of individual strata may be affirmed also of 
groups of such strata. A thick mass of sandstone will be found as a 
rule to be more continuous than one of conglomerate, but less so 
than one of shale. A series of limestone-beds usually stretches 
further than either arenaceous or argillaceous sediments. But 
even to the most extensive stratum or group of strata there must be a 
limit. It must end off and give place to others, either suddenly, as 
a bank of shingle is succeeded by the sheet of sand heaped against 
its base, or, as is more usual, very gradually, by insensibly passing 
into other strata on all sides. 
Great variations in the character of stratified rocks may fre- 
quently be observed in passing from one part of a country to another 
along the outcrop of the same rocks. Thus at one end we may meet 
with a thick series of sandstones which, traced in a certain direction, 

















































































Fic. 210.—SEcTION TO ILLUSTRATE THE GREAT LITHOLOGICAL DIFFERENCES OF 
CONTEMPORANEOUS DEPOSITS OCCUPYING THE SAME HORIZON. 
a, conglomerate; b, sandstone; c, shale; d, limestone. 
may be found passing into shales (Fig. 210). A group of strata 
may consist of massive conglomerates at one locality, and may 
graduate into fine fissile flagstones in another. A thick mass of clay 
may be found to alternate more and more with shelly sands as it is 
traced outward, until it loses its argillaceous nature altogether. 





Fic. 211.—SEcTION NEAR BRISTOL TO SHOW HOW CONGLOMERATE MAY PASS INTO 
CLAY ALONG THE SAME HoRIZON. 
B, Blaize Castle Hill; s, Mount Skitham (B.). 
Interesting illustrations of such arrangements occur in the south-west 
of England, where what are now groups of hills, like the Mendip, 
Malvern, and other eminences, formerly existed as islands in the 
Mesozoic sea. De la Beche pointed out that the upturned Car- 
boniferous limestone (a a in Fig. 211) has formed the shore against 
