Geological Papers. 57 



the sandy bottoms or on the mud flats beyond, for the fossil re- 

 mains of a few of them have been found in the indurated 

 rocks formed of this sand and mud. Besides this direct evi- 

 dence, indirect evidence of life having flourished in these seas 

 and in fresh-water swamps on the flanks of the mountains is 

 abundant. Beds of graphite are not uncommon, which possibly 

 may represent the metamorphosed peat of the swamps. Beds 

 of limestone occur, and these are usually considered as being 

 proof of the previous existence of marine life, with skeletons 

 of carbonate of lime. Then, too, beds of iron ore of great 

 thickness are found interstratified with the debris of these 

 ancient mountains. Iron ore is deposited from solution 

 through the chemical action of organic compounds set free in 

 the decay of the tissues of plants and animals. Probably bac- 

 teria helped in the deposition of the ore, either by causing 

 organisms to decay or by robbing compounds of iron of all the 

 other elements except oxygen. Copper and silver were pre- 

 cipitated in the same sand and mud flats and concentrated 

 later, especially where Lake Superior now lies, possibly by the 

 same organic reagents or by bacteria. 



DRY LAND INCREASED IN AREA. 



The development of the earth's topographic features has 

 always been hastened and emphasized by periods of mountain- 

 making. Each great range of mountains was thrown up after 

 millions of years of comparative stability of the earth's crust. 

 It was once believed that the great ranges of mountains came 

 up in a few weeks, or, at most, in a few years, but it is now 

 known that they require thousands, probably millions, of years 

 to reach maturity. 



The most ancient mountains known to the geologist, the ones 

 already described, were forced above the general level at the 

 close of the Archeozoic era. Then followed the fifteen or 

 twenty million years of erosion and deposition. The interior 

 of the earth continued to shrink very slowly because of loss of 

 heat, while the sand, clay and calcareous mud and the various 

 ores and organic compounds were being deposited in the seas 

 bordering the ancient mountains, or in swamps on their flanks, 

 till finally the accumulated stresses in the crust of the earth 

 compelled it to wrinkle, and thus enabled it to rest on the 

 smaller interior. The wrinkles followed lines of weakness, and 

 these have been shown over and over again to be along ancient 



