470 POPULAR SCIENCE MONTHLY. 



accomplished. It is to be hoped that our observations have made 

 plain the processes of rock disintegration and water transportation; 

 that in the oceans all these materials are eventually deposited in beds 

 horizontally arranged, composed of such products of decay in the 

 condition of sand and mud. We have only to point out the proof 

 that great land masses, composed of water-deposited materials, have 

 been lifted from the ocean to become continents and mountain ranges. 



As the ocean deposits slowly accumulate in layers to beds of 

 many thousands of feet in thickness, the lower parts are gradually 

 subjected to greatly increased pressure j^roduced by the overlying 

 beds. During this time waters of a varying temperature, carrying, 

 chemically dissolved, great quantities of lime, silica, and iron oxide, 

 are allowed free circulation through them. These conditions promote 

 chemical change: much silica (the mineral quartz), lesser amounts 

 of carbonate of lime (the mineral calcite), and iron oxide are precipi- 

 tated about the loose sand grains, firmly cementing them together into 

 a solid rock. A cycle has thus been completed; the dense rocks 

 composing a continent have passed by the process of weathering into 

 incoherent sand and clay, which, when transported to the ocean 

 floor, become again converted into solid rock. 



Historical records prove that during the last three thousand years 

 there have taken place many changes in the ocean's level. Old islands 

 have disappeared; new ones have emerged above the surface of the 

 water. Great stretches of seacoast exist at the present time which 

 within the historical period have been covered by the ocean. Even 

 at the present writing we are witnessing the gradual submergence of 

 some parts of the earth and the rising of others ; terraces on the north- 

 ern Atlantic coast may be seen along the hillsides many feet above 

 the present level of the ocean — all of which go to show that the rela- 

 tionship of the land to the water is an unstable one. These are the 

 evidences of continental growth and depressions from the historical 

 standpoint, and the validity of the data upon which the belief is 

 founded can not be shaken. The evidence from the geological side is 

 -overwhelming, but before we speak of this it will be well once more 

 to say a word as to the causes of continental uplift. 



From an original fluid globe possessing a high temperature, the 

 earth has now cooled down to a degree sufficiently low to permit the 

 formation of a thick rock crust. Underneath this crust an approach 

 to the old surface temperatures is still maintained, and the existence 

 of a certain degree of fluidity is demonstrated to us from time to 

 time by the phenomenon of volcanism. Successive zones of cooling 

 took place. The outer part could only conform to a shrinking interior 

 by wrinkling, folding, or bodily lifting considerable areas above the 

 general level. An adjustment of strains thus set up would take place 



