
* 
PartI.] | CONTEMPORANEOUS EROSION. 481 
from accumulating in contemporaneous water-channels, or, while still 
in the condition of soft bog-like vegetation, has been eroded by 
streamlets flowing through it.1. A section drawn across such a buried 
ehannel exhibits the structure represented in Fig. 192, wherea bed of 
fire-clay (e), full of roots and evidently an old soil, supports a bed of 
coal (d) and of shale (¢), which, during the deposition of this series of 
strata, have been cut out into a channel at f/f <A deposition of sand 
(b) has then filled up the excavation, and a layer of mud (a) has 
covered up the whole. 











Fig, 192.—SEcTION oF A CHANNEL IN A COAL-SEAM (B.), 
Currents of very unequal force and transporting power may 
alternate in such a way that after fine silt has for some time 
been accumulated, coarse shingle may next be swept along, and may 
be so irregularly bedded with the softer strata as to simulate the be- 
haviour of an intrusive rock (Fig. 193). The section (Fig. 194), 
taken by De la Beche from a cliff of Coal-measures on the coast of 
Pembrokeshire, shows a deposit of shale (a) that during the course 
of its formation was eroded by a channel at 6, into which sand was 















Fic. 193.—IrrecuLar Breppinc or Coarse AND FINE LOWER SILURIAN DETRITUS. 
FLANKS OF Guiypyr, N.E. or SNowpon (B.). 
carried; after which, the deposit of fine mud recommenced, and 
similar shale as before was laid down upon the top of the sandy 
layer, until, by a more potent current, the shale deposit was cut 
away on the left side of the section and a series of sand _ beds (¢) 
_ was laid down upon its eroded edges, An interruption of this kind, 
however, may not seriously disturb the earlier conditions of a 
deposit which, as shown in the same section, may be again resumed, 
1 Buddle, Geol. Trans. vi. (1842), p. 215, 
2 De la Beche, Geol. Observer, p. 533. 
