122 THE LAPSE OF TIME. 



and curious reptiles, and beetles and winged insects of 

 great size and beauty ; while fish replenished the waters, 

 along with an infinity of shells and corals, and other 

 inhabitants of the deep. Yet these sixteen thousand 

 feet, these eighty successive forests, these hundred and 

 sixty changes, comprise but a small fraction of the whole 

 known succession of strata. 



It is true that different strata not only may, but must 

 have been forming at one and the same time in different 

 parts of the world. But when one stratum has been 

 formed out of the wreck of another, it is self-evident 

 that they cannot have been formed together. The same 

 thing is obvious in regard to any number of layers 

 found lying in undisturbed succession one above the 

 other. They must have been formed successively, the 

 lowest first, the highest last. But one point about them 

 is far from obvious, namely, the length of the interval 

 that may have intervened between the end of one forma- 

 tion and the beginning of another. The great African 

 desert has been the great African desert as far back as 

 human histories extend ; yet in times geologically recent 

 it lay beneath the waters of the ocean. Should it be 

 again submerged before any fertilizing agencies have 

 covered it with signs of its sub-aerial exposure, another 

 layer of sand may be thrown down upon it, containing 

 new marine fossils, and no memorial be left to the future 

 geologist of the vast era during which its kindly influ- 

 ence was warming the winds of Europe, and saving us 

 from a glacial climate. The ground you stand on is 

 passing through such an interval. It was under the sea 



