200 NATURE FOR ITS OWN SAKE 
the sedimental deposits ; doubtless, there were 
at other times great streams of hot lava forced 
up from below by volcanoes and spread over 
Formation | vast surfaces, cementing the sedimental de- 
of te erust.| nosits into rock strata; and doubtless, again, 
chemical action and change by and through 
air, water, fire, produced other rock masses 
with which geology acquaints us. The forms of 
rocks, their twisted, broken, or waving strata, 
were caused by convulsions of the earth (either 
expansions or contractions of the crust) which, 
following the form of a wrinkle or a fold, have 
heaved the surface in some places and depressed 
it in other places. The deposits which we call 
soil, together with the bowlders and loose 
stones, are but the grit from rock formations, 
broken away by frost, wind, and rain, and 
washed down into the valleys by the brooks and 
streams. 
The human being has always had a very keen 
appreciation of the earth’s volume and sub- 
Solidity of |stance. Earthquakes may shake his house, but 
hecarth. | ot his faith. The tremor is but temporary ; he 
still believes in the solidity of the earth under his 
feet. And yet how seldom he thinks of the im- 
mensity of the structure, its continuity, its long 
endurance, factors which have made possible its 
