UNDERMIN-ING OF EOCKS. 153 



CHAPTEE XIX. 



UNDERMINING OP ROCKS. — VARIED ASPECT OF CLIFFS. — PLATFORMS AT THEIR 

 BASES. — RESISTANCE OP THE COASTS. — BREAKWATERS FORMED BY THE RUBBISH. 

 — HELIGOLAND,— DESTRUCTION OF LOW SHORES. . 



All the rocky promontories exposed to the violence of storms, or 

 simply washed by a current, are undermined at their base. The 

 wearing away is accomplished in a more or less rapid manner, ac- 

 cording to the progress of the waves, the distribution and inclination 

 of the strata, the hardness of the rocks, and their chemical composi- 

 tion. The method of destruction depends at the same time on various 

 hydrological and geological conditions. Strange as this assertion 

 may appear, the water of the sea can even in certain cases destroy the 

 rocks on its borders by combustion. Thus, the cliiFs of Ballybunion, 

 on the western coast of Ireland, long presented the appearance of a 

 rampart of smoking lava. Those rocks which the waves of the 

 Atlantic have pierced with grottoes, and sculptured in massive and 

 fantastic forms, having one day fallen down very extensively, the 

 alum and iron pyrites, which are contained in considerable proportion 

 in the rocks, were exposed to the action of the atmosphere and the 

 sea- water. A rapid oxydation took place, and produced a heat suffi- 

 ciently intense to set the whole cliff on fire. For weeks the rocks 

 were burning like a vast coal fire, and masses of vapour and smoke 

 rose like clouds above the high wall besieged by the surf. Scattered 

 around the space where the fire had prevailed, a heap of melted 

 scorise, and clay transformed into brick by the violence of the fire, 

 was to be seen. 



Such is the diversity of destructive agents emploj^ed by nature, 

 that, as we can easily understand, the aspect and form of the rocky 

 coasts varies likewise in a remarkable manner. Thus the cliffs of 

 England and Normandy, which are composed of somewhat friable 

 rocks, fall when their lower strata are eaten away, and their sides being 

 occasionally interrupted by '' valleuses" (narrow openings where tem- 

 porary or permanent brooks flow), they resemble enormous walls from 

 150 to 300 feet high. In the islands of the Baltic Sea, the chalky 



