410 THE EVOLUTION OF MAN. 



purpose we may most advantageously apply the method 

 which geologists have long used in determining the order 

 of the sedimentary rocks in the crust of the earth. Most 

 people know that the solid crust of our globe, a thin shell 

 which surrounds the glowing and fluid main mass in its 

 interior, consists of two chief classes of rocks : firstly, the 

 so-called Volcanic, or Plutonic rocks, produced directly by 

 the solidification of the molten internal mass of the earth 

 upon the surface ; and, secondly, the so-called Nej^tunian, or 

 Sedimentary rocks, produced from the former by the trans- 

 forming agency of water, and deposited, in stratified layers, 

 under water. At first, each of these Neptunian layers 

 formed a stratum of soft mud ; but in the course of 

 thousands of years they solidified into firm, hard masses of 

 rock (sandstone, marl, chalk, etc.), at the same time perma- 

 nently enclosing in their own mass such hard and imperish- 

 al)le bodies as had found their way into the soft mud. 

 Among the bodies, which were in this way either actually 

 fossilized, or left the characteristic imprints of their forms 

 in the soft clay, the harder parts of the animals and plants 

 which lived and died on the spot during the stratification of 

 mud are especially frequent. 



Each Neptunian rock-stratum contains its own charac- 

 teristic fossils — the remains of such animals and plants as 

 lived during that particular epoch of the earth's history. 

 By comparing these strata, it is possible to review the whole 

 connected series of earth-periods. All geologists are now 

 agreed that such a positive, historical series of rock forma- 

 tions is demonstrable, and that the lowest of these strata 

 were deposited in primaeval times, the upper in the most 

 I'ecent times. But in no one place on the surface of the 



