Part 1] GROUPS OF STRATA. 499 
- the idea of prolonged but intermittent subsidence. Let us suppose a 
- downward movement to commence, and to depress successive sheets of 
gravel, shingle, sand, and other shallow water accumulations, derived 
from the erosion of neighbouring land. If the depression be com- 
paratively rapid, the bottom may soon be carried beyond the reach of 
at least the coarser kinds of sediment, and marine lime-secreting 
organisms may afterwards begin to form a calcareous floor beneath the 
sea. Let us imagine further, that the subsidence ceases for a time, 
and that by the accumulation of organic remains and partly also by 
the deposit of fine muddy sediment, the water is shallowed. With this 
gradual change of depth, the coarser detritus begins once more to be 
able to stretch seawards, and to overspread the limestones, which, under 
the altered circumstances, cease to be formed. A gradual silting up of 
the area takes place, marked by beds of sand and mud, until a renewal 
of the subsidence, either suddenly or slowly, restores the previous 
depth and clearness of water, and allows either the old marine 
organisms, which had been driven off, or their modified descendants 
to reoccupy the area and build new limestone. 
Groups of Strata.-~Passing from individual strata to large 
masses of stratified rock, the geologist finds it needful for convenience 
of reference to subdivide these into groups. He avails himself of two 
bases of classification --(1) lithological characters, and (2) organic 
remains. 
1. The subdivision of stratified rocks into groups according to their 
mineral aspect is an obvious and easily applied classification. More- 
over, it often serves to connect together rocks formed continuously 
in certain circumstances which differed from those under which the 
strata above and below were laid down—so that it expresses natural 
and original subdivisions of strata. In the middle of the English 
Carboniferous system of rocks, for example, a zone of sandy and 
pebbly beds occurs, known as the Millstone Grit. No abrupt and 
sharp line can be drawn between these strata and those above and 
below them. They shade upward and downward into the beds 
between which they lie. Yet they form a conspicuous belt, traceable 
for many miles by the scenery to which it gives rise. The red rocks 
of central England, with their red sandstones, marls, rock-salt, and 
gypsum, form likewise a well-marked group or rather series of 
groups. It is obvious, however, that characters of this kind, though 
sometimes wonderfully persistent over wide tracts of country, must 
‘be at best but local. The physical conditions of deposit must always 
have been limited in extent. A group of strata showing great 
_ thickness in one region will be found to die away as it is traced into 
another. Or its place is gradually taken by another group which, 
even if geologically contemporaneous, possesses totally different 
lithological charactérs. Just asat the present time a group of sandy 
deposits gradually gives place along the sea-floor to others of mud, 
and these to others of shells or of gravel, so in former geological 
periods contemporaneous deposits were not always pea 
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