THE GEOLOGY OF PLYMOUTH. 
465 
also, as on the north of the limestone, there has heen repetition, 
and that the actual thickness of the series is much less than the 
apparent. 
Perplexing as these rocks are, if we endeavour to indicate the 
exact relations of the different members of the series, it is not so 
difficult to recall the causes which issued in their formation. A 
continuance of that downward movement of the ocean bed which 
accompanied the building of the reef, combined with changes in 
the character of the detrital matter brought down by the rivers of 
the adjacent coast line, and subject to volcanic interference, will 
supply all the conditions required. The beds of limestone detached 
from the main body of the reef clearly indicate that whatever the 
causes which led to the disappearance of the coral animals they 
were not of instant effect. There was no great cataclysm, no 
sudden convulsion of nature. The ash and trap beds need no ex- 
planation. The grits and sandstones point to a change in the 
character of the matter brought down by the neighbouring river 
or rivers. The sandstones of "Withy Edge show that the arena- 
ceous influences, although gradual in their appearance, at length 
long predominated. The ripple-marked beds of Eovisand prove 
that when they were formed the waters must have been very 
shallow indeed. 
Differences in the character of deposited rocks necessarily imply 
variations in time or changes in operation. All round the shores 
of the Sound may be seen the rocks of the future in process of 
construction — here a mudbank, there a sandy flat ; here a layer of 
shingle, there a beach thickly charged with shells and the remains 
of other organisms ; here again a bare water- worn rock, with sand 
or pebbles in its hollows. But all these formations that differ so 
widely are strictly contemporaneous, strictly the effects of the 
same system of causation. Take another illustration. Go up one 
of our Devonshire hillside lanes after heavy rain. You will see 
there a little gully, the bed of a miniature rain torrent. Where 
the road is steepest the gully will be worn and deep, cut down to 
the rock. If the road approaches a level the gully will expand, 
and much of the detritus brought down from above be deposited — 
the heavier particles first, and then, as the stream loses force, the 
lighter. And thus one river may have borne into this ancient sea 
either silt, or sand, or gravel, and shale, or sandstone, or conglomerate 
result. 
