TIME AND CHANGE 



torn, now undreamed of, and in volcanic eruptions 

 as great as any in the past. Such a shrinkage and 

 eruption made the Hawaiian Islands, probably in 

 Tertiary times; such a shrinkage may make other 

 islands and other continents before another period 

 of equal time has elapsed. 



Of course the periods and eras into which the 

 geologists divide geologic time are as arbitrary as 

 the months and seasons into which we divide our 

 year, and they fade out into each other in much the 

 same way; but they are really as marked as our 

 seasonal divisions. Not in their climates — for the 

 climate of the globe seems to have been uniformly 

 warm from pole to pole, without climatic zones, 

 throughout the vast stretch of Palaeozoic and Mes- 

 ozoic times — but in the succession of animal and 

 vegetable life which they show. The rocks are the 

 cemeteries of the different forms of life that have 

 appeared upon the globe, and here the geologist 

 reads their succession in time, and assigns them to 

 his geologic horizons accordingly. The same or allied 

 forms appeared upon all parts of the earth at ap- 

 proximately the same time, so that he can trace his 

 different formations around the world by the fossils 

 they hold. Each period had its dominant forms. 

 The Silurian was the great age of trilobites; the 

 Devonian, the age of fishes; Mesozoic times swarm 

 with the gigantic reptiles; and in Tertiary times 

 the mammals are dominant. Each period and era 



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