THE HARDENING OF ROCKS 



air fed by the constant transference of vapour from the 

 earth to the skies, and the condensation of the vapour to 

 rain, falling again on the earth, were greatly magnified. 

 Thus the rocks of the earth, some of them only cooling 

 and not yet hardened, were subjected to " weathering" of 

 a kind of which it is hard to form any sufficient idea. 

 The key to all geology is that what is going on now on 

 the earth is similar to what always has been happening, 

 (differing in degree rather than in kind), and that conse- 

 quently the rocks of millions of years ago were washed by 

 rivers down to the lower levels and were deposited as sedi- 

 ment in streams, in lakes, and in the sea. Thus the age of 

 the " sedimentary rocks " began while the earth was still 

 too warm to preserve any vestiges of life. 



Earthquakes much more violent than now and volcanic 

 outbursts often upset the steady order of things, but the 

 earth was settling down. During this settling-down 

 process rocks, as we have seen, were being formed by 

 deposits; but they were very liable still to be invaded 

 by bursts of volcanic activity from the inner cauldron of 

 the earth, and they were very apt to be twisted out 

 of their regular shape by great earth movements. They 

 were also liable to be baked by the neighbourhood of the 

 restless, unconfined molten rocks, nearer then to the surface 

 than now. Geologists call the great period of time when 

 all the rocks continually flowed out on to the surface of 

 the earth, and were, in fact, all molten before they solidi- 

 fied, the Archaean Era (from a Greek word signifying the 

 beginning). Next in order to these rocks are those which 

 were laid down in the agitated times when the earth was 

 i 129 



