Fossils 27 



material under water. We. must further agree that the 

 lower layers were deposited before the upper layers, that 

 each layer is older than those above it and younger than those 

 underneath. 



When we compare series of strata in one part of the 

 world, or even of the same country, with series in another 

 region, it becomes often impossible to establish any kind of 

 correspondence. Layers of coal in Pennsylvania, for ex- 

 ample, we may believe to be older than the rocks found above 

 them and more recent than the underlying rocks; but how 

 can we tell whether they belong to the same period as coal 

 layers in Indiana or in England? 



Another difficulty in interpreting the relative age of 

 the earth's layers is met when the strata are not perfectly 

 horizontal. Again and again we find layers tilted at every 

 possible angle. Now the geologists find good reasons for 

 assuming that these tilted layers were originally horizontal 

 and were forced into their existing positions by a slow or rapid 

 upthrust from below. The crust of the earth has become 

 wrinkled, chains of mountains having been pushed out. Or 

 sudden eruptions, like volcanic outbursts, have pushed the 

 crust up. The continuity of strata can often be traced on 

 opposite sides of such outbreaks. In other cases the record 

 has been marred by the action of the weather and of waters. 

 In some cases masses have been completely turned over, so 

 that the layers lie in an order opposite to that in which they 

 were originally put down. These and other irregularities 

 have made it impossible in the past for the geologist to read 

 the rocks as directly and clearly as we should like, especially 

 when comparing structures widely separated in space, or on 

 different continents. 



Fossils 



A great aid to reconciling the scattered facts was found 

 in the observation made by an English surveyor in the Nine- 

 teenth' Century. William Smith, who traveled extensively 



