14 



HISTORY OF C OH ASSET. 



All the superimposed rock must have been worn off at 

 a rate less than one one-hundredth part of an inch each 

 year in order to lose two or three miles of it in twenty 

 five million years. 



The oxygen and other gases of the air are continually 

 decomposing even so solid a rock as granite. The frost 

 gets its teeth between the crystals and bites off pieces. 

 The rain soaks into it and softens it and the sun blisters it. 

 Every stream of water that has a particle of mud in it 

 is scurrying away to the sea with tidbits stolen from the 

 hills. 



All they need to have carried or corroded each year for 

 twenty-five million years is the mere thickness of a hair 

 in order to have scoured off two or three miles of rock 

 from the top of our granite. 



But the rock which lay on top was not so stubborn and 

 hard as our granite ; and it probably surrendered to the 

 army of destruction much more rapidly than our granite 

 ledges are decaying. 



Besides the slate already referred to, there were layers 

 of pudding stone. These layers of conglomerate were 

 nothing but stones and sand and clay which were hardened 

 into rock by layers of lava that oozed out time and again 

 from beneath, through cracks in the crust. 



It is quite plain that the granite must be very much 

 younger than the rock under which it formed, because 

 that rock had to accumulate to a great thickness before 

 the granite could begin. The first layers were under 

 water, and were deposited just as clay and sand are now 

 deposited, from farther inland upon the sea bottom, what- 

 ever that was ; and it hardened into rock when there was 

 enough of it to press hard or when other formations 

 pressed it down where the internal heat could affect it. 



The effect of this internal heat was enough to destroy 

 wholly the sea bottom upon which the Cambrian slate 

 was deposited. 



