Ch. XI.] 
BRITISH ALLUVIUMS. 
147 
been caused by earthquakes or excavated by the power of 
running water during the rise of the land. The alluvium in 
those more modern valleys would consist partly of pebbles 
washed out of the older gravel before mentioned, but chiefly of 
fragments derived from the wreck of those rocks which were 
removed during the erosion of the valleys. 
Many of the most widely distributed of the British allu- 
viums may we think be referred to the action of the sea 
previous to the elevation of the land ; and for this reason we 
never expect to be able to trace all the pebbles to their parent 
rocks. If it be objected that the high antiquity thus ascribed 
to many of our superficial deposits seems inconsistent with their 
actual state of preservation, we may observe, that they are 
often composed of indestructible materials, such as flint and 
quartz, and in many cases they have been protected for ages 
from the corroding action of the atmosphere by an envelope of 
loam or clay, from which they have been partially and slowly 
washed out by rain. 
It must not, however, be understood that we refer the 
greater part of the alluviums scattered over our continents to 
the waves and currents of the sea, but merely some of those 
which have been justly regarded as most singular and anoma- 
lous, both in position and in the discordance of their contents 
with any known rocks in the adjacent countries. 
Grooved surface of rocks. — We sometimes find the surface 
of large tracts hollowed out extensively in parallel grooves, 
such as have been described by Sir James Hall on the summits 
of the Corstorphine Hills, where I have myself examined them, 
in company with Dr. Buckland. These grooves may have 
been caused by the friction of blocks rolled along the floor of 
the ocean before the country emerged from the deep. The 
same appearances may be seen on a smaller *scale, in the beds 
of many mountain- streams in Scotland, and I observed them 
strikingly displayed on Etna, in the defile called the Portella 
di Calanna, where a hard blue lava of modern date has been 
furrowed in this manner by the rolling of blocks down a steep 
declivity. L 2 
