GEOLOGICAL TIME 415 



mate, however, must always have been zonal, as it now is. Fur- 

 thermore the earth's crust was not uniformly constructed nor is 

 it uniformly constituted, as is shown by its stratification. It be- 

 comes necessary therefore to segment geological time into intervals 

 to indicate the periods during which the different strata were 

 formed. It would also be anticipated that under these conditions 

 the same kinds of strata would not be encountered everywhere 

 that examination was made of a vertical section of the earth's 



crust. 



The student must also be prepared to accept the conclusion 

 that the same kinds of strata do not occur everywhere in the same 

 relative positions. Evidence is furnished by exposed rocks in 

 such situations as mountainsides, canyon walls, mine shafts, escarp- 

 ments, and tunnels. Here the strata may be observed to be vari- 

 ously folded, buckled, and jumbled. Moreover, sedimentary 

 rocks are found in some places to be deeply covered by basaltic 

 lava and volcanic ash. In certain localities layers of rock have 

 slipped past each other and been reshuffled in the reverse order of 

 that in which they were formed. In others great beds of coal, 

 lignite, or peat occur. Extensive deposits of salt, sulphur, gypsum, 

 limestone, phosphate, and various minerals are found in other 

 localities. There is evidence that certain parts of the earth were 

 inundated for long periods and that long ago these areas were 

 raised up out of the sea. Faunas and floras existed that were very 

 different from those present anywhere today. Catastrophic 

 changes in climate evidently occurred. The earth's crust must 

 have been in convulsion when the mountains were formed. It is 

 from evidence of this kind, gathered from various localities, that 

 geologists have been able to piece together and to formulate a 

 plausible conception of the sequence of geological events and to 

 approximate the duration of the different segments of geological 



time. 



Estimates of the age of the earth do not agree closely, partly 

 because they are based upon different kinds of evidence. From 

 Biblical evidence Archbishop Ussher placed the age of the earth 

 at approximately 4000 years. If calculations of the duration of 

 geological time are based upon the rate of dissipation of the earth's 

 initial store of heat energy, however, a figure of about 100 million 

 years is deduced. 



