‘Parri.] § PERSISTENCE OF STRATA. 491 

tion sprang up upon it. This appears to have taken place in wide 
shallow lagoon-like expansions of the sea, bordering land clothed with 
dense vegetation, and to have been accompanied by slow, intermittent 
but prolonged subsidence of the sea-bottom. Hence, during pauses 









10 
Fig. 208.—Srction oF STRATA FROM THE BASE OF THE LIAS TO THE Tor or THE 
Trias, SHEPTON MAuuet (B.). . 
a, Grey Lias limestone and marls; b, earthy whitish limestone and marls; ¢, earthy 
white limestone; d, arenaceous limestone; /, grey marls; g, red marls; h, sand- 
stone with calcareous cement; 7, blue marl; #, red marl; J, blue marl; m, red 
marls. 
of the downward movement, when the water shoaled, an abundant 
erowth of water-loving or marshy plants sprang up on the muddy 
bottom, somewhat like the mangrove swamps of the present day, and 
continued to flourish until the muddy soil was exhausted, or until 
subsidence recommenced and the matted jungles, carried under the 
water, were buried under fresh inroads of sand or mud. Every coal- 
field contains a succession of buried forests with a constant repetition 
of the same kinds of intervening strata (Fig. 209). 
For obvious reasons conglomerate and sandstone occur together 
rather than conglomerate and shale. ‘The agitation of the water 
which could form and deposit coarse detritus, like that composing 
conglomerate, was too great to admit of the accumulation of fine silt. 
On the other hand, we may look for shale or clay rather than sand- 
stone as an accompaniment of limestone, inasmuch as when the gentle 
currents by which fine argillaceous silt was carried in suspension 
ceased, they would be succeeded by intervals of quiet clearing of the 
water, during which calcareous material might be elaborated either 
chemically or by the action of living organisms. 
Relative persistence of Strata.—A little reflection will con- 
vince the student that all sedimentary rocks must thin out and dis- 
appear, and that even the most persistent, when regarded on the 
ereat scale, are local and lenticular accumulations. Derived from 
the degradation of land, they have always accumulated near land. 
They are necessarily thickest In mass as well as coarsest in texture 
nearest to the source of supply, and become more attenuated and 
fine-grained as they recede from it. We have only to observe what 
1 Sterry Hunt has called attention to the fact that the underclays of the Coal-measures 
have generally been deprived of their alkalies by the vegetable growth which they 
supported. 
