70 PREHISTORIC EUROPE. 



other kinds of rock being as a rule of little consequence. Nearly 

 all owe their origin to the chemical and mechanical action of 

 the subaerial forces — rain, frost, and running-water, while a few 

 have been formed in other ways. There are some rare instances, 

 for example, where a lava-stream has flowed over and solidified 

 above a mass of snow and ice, and the subsequent melting of 

 the latter has left a hollow behind. Again, during earthquakes 

 rocks are frequently rent asunder, and when these fall rudely 

 together, irregular cavities are left between the disjointed masses. 

 Similar results often take place when great landslips occur. But 

 such cases are exceptional, and need not at present occupy our 

 attention ; the caves which are of most interest to the student 

 of Palaeolithic times are those which have been more or less 

 slowly excavated in the body of the rocks themselves. Caves 

 of this character are of two kinds ; — there are some which have 

 been formed at the surface and in the light of day, while others 

 have been hollowed out at various depths by the action of under- 

 ground water. Those of the former class are generally of 

 smaller size than the others, and are typically represented by 

 the hollows that occur at the base of many inland cliffs, and 

 by the sea -caves that are so commonly met with along the 

 present coast-line, and in the rocks at higher levels where the 

 waves and breakers in former times have been busy at their 

 work of erosion. As a rule the hollows at the base of inland 

 cliffs mark the outcrop of some softer or more easily disinte- 

 grated rock than the others with which it is associated. When 

 a hard unyielding stratum overlies a softer or more readily 

 decomposed bed, the latter will crumble away and be worn back 

 by the mere action of the weather, and hollows of this nature 

 may of course occur either at the base of a cliff or steep slope, or 

 at any intermediate level between the base and the top. Should 

 such a soft rock happen to be washed by some stream a hollow 

 of considerable size may be scooped out, and this will continue 

 to be enlarged so long as the weather acts and the water flows, 

 until the overhanging harder stratum gives way and topples 

 down, and the cave is, for the time being, obliterated. For the 



