26 THE CRUST WE DWELL UPON. 



tive ages of formations the more recent containing the 

 remains of plants and animals nearly akin to those still 

 peopling the earth, while the more ancient contain plants 

 and animals that differ widely from the existing and this 

 difference increasing with the age of the formation. 



Here, then, by means, first, of superposition, second, by 

 mineral composition, and, thirdly, by fossil remains, the 

 geologist can arrive at the relative ages of the rock-for- 

 mations that constitute the earth's crust, and can arrange 

 them into sections and systems and periods, just as the 

 historian arranges the reigns and dynasties and periods of 

 human history. As the one speaks of ancient, medieval, 

 and modern times, so the other speaks of primary, second- 

 ary, and tertiary systems. As the one groups the popula- 

 tions of the world into ancient, medieval, and modern, so 

 the other groups its life into eozoic, palaeozoic, mesozoic, and 

 cainozoic that is, dawn-life, ancient-life, middle-life, .and 

 recent-life. But while the geologist thus reads the history 

 of the earth mainly through its stratified rocks, he at the same 

 time receives important aid from its igneous or unstratified 

 masses. These, as volcanic products, break through the 

 stratified formations, throw them out of their horizontal 

 position, overflow them in part, insert themselves among 

 them as intrusive masses, and fill up rents and fissures in 

 the form of dykes and veins. All this gives ample evidence 

 of former change, and presents a lively picture of the opera- 

 tion of these gigantic forces which are still so instrumental 

 in modifying the existing aspects of our planet. In fine, 

 the whole crust is replete with evidence, physical and vital, 

 of the earth's former conditions. Our globe writes, as it 

 were, her own history every layer of mud and sand laid 

 down in water, every shower of ashes or sheet of lava 

 ejected from a volcano, every stem and twig, every shell 

 and tooth and scale preserved in sediment, forming an 



