30 A NATURALIST IN TEE GREAT LAKES REGION 



fragmentary bits, a clay tablet dug up from some old ruin in 

 the Euphrates Valley, some bit of papyrus taken from an 

 Eg}^tian pyramid, some roll unearthed from ash-covered 

 Herculaneum, so must this geological record be constructed 

 out of authentic scraps of information gathered from the rocks 

 of the several continents. Much of the record is covered up 

 under layers of soil and is inaccessible. Only in the quarry, the 

 mine, the chance exposure of rock strata by railroad cut, the river 

 gorge, or mountain ledge can the explorer find the fossil remains. 

 These give opportunity for the study of a mere fraction of the 

 whole record. It is surprising that the geologist has been so 

 successful in reconstructing the history^ of hfe upon the earth in 

 the face of such difhculties. 



The historian divides the time of the enactment of human 

 histor}^ into the modern age, the medieval, the ancient, back 

 still farther, an age of myth and folklore in which events are 

 dimly seen, and finally a prehistoric age when the happenings 

 must be matters largely of conjecture. So the geologist sub- 

 divides the history which he reads in the strata of the earth's 

 crust into the Cenozoic, ^lesozoic. Paleozoic, Proterozoic, and 

 Archaeozoic eras. Then just as man subdivides historical ages 

 into smaller periods, as for instance the age of ancient history, 

 into the Babylonian period, the Egyptian period, the Grecian 

 and Roman periods, so the geological eras are subdivided. The 

 tabulation at the end of the chapter will show the main divisions 

 and subdivisions of geological time and will serve as reference 

 to place the events about to be recorded. It will also facilitate 

 the reading of the bulletins in the bibliography at the end of the 

 book. 



The different rock strata are referred by the experts to various 

 of these time divisions according to the time of their formation. 

 Thus we speak of carboniferous rocks — those that were deposited 

 during the Carboniferous period. The relative age of any rock 

 layer is determined by finding what rocks are below it, what 

 are above it, and by the character of the fossils it contains. 



