

x 
Pane iE}. OVERLAP. 495 
corals, crinoids, molluscs, &c., whose remains are now to be seen in 
intercalations of crinoidal limestone. 
Overlap.—Sediment laid down in a subsiding region wherein 
the area of deposit is gradually increased, spreads over a progressively 
augmenting surface. Under such circumstances, the later portions 
of a formation or series of sedimentary accumulations will extend 
beyond the limits of the older parts, and will repose directly upon 
the shelving bottom, with none of these older strata underneath 
them. This relation, called Overlap (Fig. 213), in which the higher 
or newer members are said to “overlap” the older, may often be 
detected among formations of all geological ages. It brings before 
us the shore-line of ancient land-surfaces, and shows how, as these 
sank under water, the gravels, sands, and silts gradually advanced 
and covered them. 

Fie. 213.—SEcTION OF OVERLAP IN THE LOWER JURASSIC SERIES OF THE 
SOUTH-WEST OF ENGLAND (B.). 
The Old Red Sandstone (¢), Lower Limestone Shale (4), and Carboniferous Limestone 
(a) having been previously upraised and denuded, the older beaches (d m) laid 
down upon them were successively covered by conformable Jurassic beds. The 
Lias (e), with its upper sands (f), is overlapped by the extension of the Inferior 
Oolite (g) completely across their edges until this formation comes to rest directly 
on the Paleozoic strata at n.. The corresponding extension of the overlying Fuller’s 
earth (h 1) and limestone (2) has been removed by denudation. 
Relative Lapse of Time represented by Strata and by the 
Intervals between them.—Of the absolute length of time repre- 
sented by any strata or groups of strata no satisfactory estimates 
have yet been possible. Certain general conclusions may indeed be 
drawn, and comparisons may be made between different series of 
rocks. Sandstones full of false-bedding were probably accumulated 
more rapidly than finely-laminated shales or clays. It is not un- 
common in certain Carboniferous sandstones to find huge sigillarioid 
and coniferous trunks imbedded in upright or inclined positions. 
Where, as in Fig. 214, the trees actually grew on the spot where 
their stems remain, it is evident that the rate of deposit of the sedi- 
ment which entombed them must have been sufficiently rapid to 
have allowed a mass of twenty or thirty feet to accumulate betore 
the decay of the wood. , Of the durability of these ancient trees we 
of course know nothing; though modern instances are on record 
where, under certain circumstances, submerged trees may last for 
some centuries. We may conjecture that where stems are enveloped 
in one continuous stratum, the rate of accumulation was probably, on 
the whole, somewhat rapid. The general character of the strata 
among which such erect tree trunks occur obviously indicates ex- 
1 De la Beche, “ Geol, Observer,” p. 485. 
