390 THE CHANGING GENERATIONS 



In successive periods of deformation first one and then another such 

 trough gave way, crumpling into mountains in such a manner that the 

 sedimentary strata bent into corrugated folds or in places even broke 

 and slid over one another. Where the pressure and the accompanying 

 heat were intense, the nature of the rocks was greatly altered, becoming 

 more or less crystalline, with the formation of new minerals and the 

 destruction of all fossils. Thus shales or mud rocks were changed to slates, 

 limestones to marbles, and sandstones to a much denser kind of rock 

 called quartzite. Any rock, whether originally sedimentary or igneous, 

 which has been thus altered by pressure and heat is called a metamorphic 

 rock. Where the mountain-making forces acted less vigorously, the sedi- 

 mentary rocks were thrown into folds without losing their sedimentary 

 characteristics and without destruction of fossils. 



Not all sediments have been laid down in the seas, though this is where 

 the greatest accumulations have occurred. They may be deposited in 

 lake basins or in swamps or as river deposits built up like land deltas 

 on the plains at the foot of mountains. Much of our knowledge of the 

 later steps in the evolution of life comes from fossils found in fresh-water 

 and land deposits. From the geological standpoint, however, such sedi- 

 ments are far less important than the marine deposits because of their 

 merely local extent and because they are so much more likely to be 

 destroyed by subsequent erosion. 



The order of the strata. In an undisturbed series of sedimentary 

 strata the lowest layers are obviously the oldest, the uppermost the 

 youngest. In regions such as Florida, where the strata lie relatively flat, 

 and because of the low elevation streams do not cut deeply into the 

 surface, only the uppermost layers are accessible to study. In such regions 

 the nature of the deeper strata can be learned only from well borings. 

 But in regions where the land has been elevated and the strata left unde- 

 formed or only gently tilted or warped, streams have often cut sections 

 through the mass of layers so that their edges are exposed in the valley 

 sides; or erosion may have stripped off parts of the surface, so that in 

 crossing a region, one can traverse the beveled edges of the layers. 



Each section thus exposed to study constitutes only a small part of 

 the entire rock record. Nevertheless, by correlating the pieces of the story 

 revealed in one area with those preserved in other areas, a fairly complete 

 record of the last quarter or third of earth history has been worked out. 

 Fossils are used for correlating distant exposures of rock strata in the 

 following manner. Each stratum is found to have a peculiar assemblage 

 of fossils different from that of other strata. The succession of these fossil 

 faunas and floras can be determined in one region where the rocks lie 

 undisturbed ; then the sequence worked out in that region can be used to 

 determine the relative ages of strata in other regions. This method is 



