PRELIMINARY OUTLINE, 



17 



between beds of stratified rock can be told from those which flowed 

 out on the surface and were subsequently buried, for in the former 



Fig. 2. — Diagrammatic representation of the relations of igneous rock to stratified 

 rock. The igneous rocks, represented in black, have been forced up from beneath. 



case the sedimentary rocks, both above and below the igneous rock, 

 were affected by the heat, while in the latter case only those below 

 were so affected. 



More commonly than otherwise the metamorphic rocks (Fig. 3) 

 lie beneath the sedimentary beds and are often broken through by the 





,,,A,. U 11.1 ;,. 



Fig. 3. The figure represents a section of the earth about 1000 miles long. The un- 

 equally thick black line at the top represents on something like its proper scale the 

 depth of the stratified rock. The area below represents crystalline rock, largely 

 metamorphic. 



igneous rocks. From their position in many places their great age 

 may be inferred, but locally, especially where dynamic action has 

 been severe, relatively young rocks are metamorphic. 



Four great sedimentary eras. — The water-laid series represents 

 four great eras in the history of the earth, as sho^vn by the relics of 

 hfe imbedded in them. Beginning with the latest, these are the Ceno- 

 zoic (recent hfe), during which the life took on its modern aspect; the 

 Mesozoic (middle life), during which the life bore a mediaeval aspect; 

 the Paleozoic (ancient hfe), during which the hfe belonged to older 

 types; and the Proterozoic (earlier life), during which it is inferred 

 that much life prevailed, though its record is very imperfect. It may 

 safely be assumed to have been more primitive than that of the Paleo- 

 zoic, as it was earlier. Each of these great divisions embraced several 

 lesser periods or epochs, and these again are subdivided more and 

 more closely according to the degrees of refinement to which studies are 

 carried. The chief of these subdivisions are given in the table on page 

 lOi, and others will come under consideration in the historical chapters. 



