THE PEIMAEY PERIODS. 



OLDER AND YOUNGER PORTIONS OF THE EARTH'S CRUST THESE 

 REPRESENTED BY PERIODS OF TIME OR SYSTEMS OF STRATA 

 TECHNICAL ARRANGEMENT OF THESE PERIODS AND SYSTEMS THE 

 PRIMARY OR EARLIEST YET KNOWN, EMBRACING THE LAURENTIAN, 



CAMBRIAN, AND SILURIAN COMPOSITION AND DISTRIBUTION OF 



THESE RESPECTIVE SYSTEMS THEIR CHARACTERISTIC ROCKS AND 

 FOSSILS GENERAL PAUCITY OF LIFE ADVANCE DURING THE 

 SILURIAN CYCLE OF CRYPTOGAMIC PLANTS AND INVERTEBRATE 

 ANIMALS GEOGRAPHICAL ASPECTS OF THE PRIMARY PERIODS 

 INTEREST ATTACHED TO THEM AS THE DAWN OF WORLD - 

 HISTORY. 



NOTHING can be clearer than this : if the higher and more 

 exposed portions of the earth be continuously wasted and 

 worn down, and the rock-material so wasted be as continu- 

 ously deposited in the receptacles of lakes and estuaries, 

 the older deposits (or formations, if you prefer to call 

 them) will be the deeper, and the newer will be arranged 

 in order of succession above them. Or put it in this 

 way: if the sediments of lakes and seas be gradually 

 converted into solid strata, and now and again upheaved 

 into dry land by vulcanic forces, the oldest, generally 

 speaking, will be the most consolidated and altered. Or 

 again, if the sediments of lakes and seas contain less or 

 more of the remains of plants and animals that have been 

 either drifted from the land or entombed in the areas where 

 they flourished, then the latest enclosed remains will be 



