GEOLOGY OF AGEICULTURE. 47 



earth about as mucb as the smallest dust would that 

 of an artificial globe. The average height of the land 

 is probably a little less, and the. average depth of the 

 ocean a little more, than two miles. 



68. The crust of the earth, thinner comparatively^ 

 there is reason to believe, than the shell of an Qgg^ 

 though certainly many miles in thickness, is solid 

 rock, covered, three-fourths, as before stated, with 

 water, and the remaining fourth, with broken rocks, 

 stones, rounded pebbles, gravel, sand, and clay, to a 

 depth of from a few inches to a few hundred feet ; the 

 whole sustaining an atmosphere supposed to be about 

 45 miles in height, and known to weigh just about 15 

 pounds to each square inch of the earth's surface. 

 The weight of air over each square foot of the earth's 

 surface is 2160 pounds ; and the weight of the whole 

 atmosphere is equal to the weight of a covering of 

 water over the entire globe 84 feet deep. This is 

 known from the action of a common suction pump, in 

 which the pressure of the atmosphere just balances a 

 column of water 34 feet high. 



STRATIFIED AND UNSTRATIPIED ROCKS. 



69. Almost every one must have noticed that some 

 rocks, as they appear in various situations exposed to 

 the eye, are formed into regular layers, or beds, rest- 

 ing one upon another. These layers are called strata, 

 and the rocks that exhibit them are said to be strati- 

 fied. Other rocks present no such appearance of 

 9traiificati:,i> — no regular layers one itpon another, and 



