132 INTRODUCTION TO EVOLUTION 



ian chain, including the Green Mountains of Vermont, the White Moun- 

 tains of New Hampshire, the Berkshires, the Alleghenies, the Blue Ridge, 

 and so on, was higher than the present Rocky Mountains. How many 

 "chapters" of the geologic record were destroyed as these mountains were 

 being worn down to their present height! 



Fortunately strata lost by erosion in one part of the country may be pre- 

 served in another region. But it is not possible to fill in all the gaps in the 

 record by comparing different regions of the earth's surface. Gaps are fre- 

 quently left in the records of animal life, since a species of animal found in 

 one part of the country at a certain time might not have been an inhabitant 

 of another part of the country at that same time. 



We may suppose that after millions of years of erosion our mountain 

 range is reduced to a row of low hills. Finally another change in the earth's 

 crust occurs and the region is dropped below sea level again, the hills be- 

 ing submerged. Then this newly formed sea floor will begin to collect de- 

 posits again, as did the floor of this region when we first began our story of 

 it. The new deposits will be laid down immediately on top of whatever 

 deposits were left by the preceding erosion. If eventually the region is 

 again lifted into the air where geologists can study it they will find the old 

 deposits, left from the preceding period of erosion, and immediately on top 

 of them the new deposits. No remains will be left of the, perhaps, thou- 

 sands of feet of deposits which were eroded away. The result is much like a 

 book which has Chapter 15 following immediately after Chapter 3. For- 

 tunately geologists are astute in detecting such unconformities in series of 

 strata, but detecting the presence of a gap does not necessarily enable one 

 to fill in the lost history. 



Another source of difficulty in interpreting the geologic record lies in the 

 fact that at times older rocks may come to lie above younger ones. Fig. 7.1 

 demonstrates graphically one way in which this comes about. The upper 

 diagram shows a series of strata deposited smoothly, as described in our 

 hypothetical example. Obviously the older layers are at the bottom of the 

 series. Then a mountain range is formed by folding of the earth's crust. 

 The fold is accompanied by a strong thrust, from the west in the example 

 diagramed, which displaces a section of the crust toward the east, ac- 

 tually sliding it over the strata already present in that region, as shown in 

 diagram 4 (Fig. 7.1 ). Such an overthrust may extend for many miles. As a 

 result older strata are found to lie over younger ones. The land surface is 

 subsequently sculptured by erosion, many of the signs of the phenomena 

 which produced the observed sequence of strata being obliterated. Pro- 

 longed study involving large areas suffices to reveal the true explanation. 



