Ch. XXII.] 
FURROAVS ON THE CHALK HOW CAUSED. 
proportion of calcareous matter. In the crag of Norfolk, 
undoubtedly, we find great heaps of broken pieces of white 
chalk, with slightly-worn and angular flints; but in this case 
we may infer that the attrition was not continued for a long 
time; whereas the large accumulations of perfectly-rolled 
shingle, which are interstratified with our Eocene formations, 
proves that they were acted upon for a protracted period by 
the waves. We have many opportunities of witnessing the 
entire demolition of the chalk on our southern coast, as at Sea- 
ford, for example, in Sussex, where large masses are, year after 
year, detached from the cliffs, and soon disappear, leaving 
nothing behind, but a great bank of flint shingle *, 
Valleys and furrows in the chalk how caused. — The furrows 
which occur on the surface of the chalk, filled with sand and 
pebbles of the plastic clay, may be easily explained if we sup- 
pose the English Eocene strata to have been formed during a 
period of local convulsion. For if portions of the secondary 
rocks emerged from the sea in the south-east of our island 
during that period, it is probable that the chalk underwent 
many oscillations of level, and that certain tracts became land 
and then sea, and then land again, so that parts of the surface, 
first excavated by currents or rivers, were occasionally sub- 
merged, and, after being covered by tertiary deposits, upraised 
again. We must also remember, that almost every part of the 
chalk must have been exposed for some time to the ? ;tion of 
the waves, if we assume the elevation to have been slow and by 
successive movements. The valleys seen everywhere on the 
surface, and the layers of partially-rolled and broken flints 
which very generally overspread it, may be referred to the sea 
breaking upon the reefs and shoals when the rocks were about 
to emerge. We apprehend, indeed, that no formidable dif- 
. ficulty will be encountered in explaining the position of the 
tertiary sand which sometimes fills rents and furrows in the 
chalk, or the occurrence of banks of shingle at the junction of 
the tertiary strata and the chalk, if we once admit that the 
* Vol. i. p. 279, and Second Edition, p. 319. 
