THE WORLD IN THE MAKING 29 



scientists of the expedition then skilfully remove them from the 

 rock, often spending days in chiseling away the rock from around 

 the cast of some old bones or in cementing together the more 

 or less cracked remains so they may be transported safely. They 

 are then shipped to the great museums, where they can be 

 leisurely set up and carefully studied. 



The paleontological record is at no place on the earth com- 

 plete. It would seem at first thought as if in digging deep mines 

 we might hope to begin excavations in rocks of recent formations 

 and then dig down deeper and deeper through successively older 

 rocks. So we could find animal and plant remains that would 

 enable us to reconstruct readily the history of living organisms. 

 But the task is not so simple. 



In an ideal section of the earth's crust (using the term in 

 the sense of the outer accessible layer) we should find the oldest 

 rocks deep down, then later and later rocks in successive layers 

 upon them, and finally the very recent rocks at the surface of 

 the earth. Such a condition is purely ideal; it exists only in 

 the mind of the geologist. It is fortunate that this is so, for if the 

 oldest rocks were always buried miles and miles deep under the 

 layers of later formations we could know nothing about them. As 

 it is they have often been upheaved and recent rocks have been 

 eroded off from them. The rock layers, by violent upheaval, 

 have been turned upside down in places so that the older rocks 

 are on top. 



Whenever the sedimentary rocks with their fossil remains have 

 been upheaved above the ocean level, the process of deposition 

 has been thereby stopped and so the fossil record has been inter- 

 rupted. Moreover, no sooner were these rocks lifted above the 

 ocean than erosion began, and the historical record that had been 

 made was rapidly destroyed. 



Thus the paleontologist reads the history of life upon the 

 earth from a great book whose leaves are the rock strata on which, 

 in fossil characters, the record is inscribed. Just as ancient 

 human history must be patched together by the experts from 



