Ch. XXII.] FURROWS ON THE CHALK HOW CAUSED. 311 



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 hoiu 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 action 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. 



