8 FROM FISH TO PHILOSOPHER 



consequently changes in continental weight follow an ir- 

 regular cycle. Climatic changes, which are themselves 

 related to the elevation of great mountain ranges, have 

 at times led to the formation of great glacial ice fields 

 that have at once lightened the oceans and added their 

 weight to that of the continents, causing the covered re- 

 gions to sink deeper into the basaltic layer. Between these 

 and other factors, mountain building has reached peak 

 activity at intervals of roughly thirty milhon years (as 

 illustrated in Figure 2), with three major episodes sepa- 

 rating the Proterozoic, Paleozoic, Mesozoic, and Ceno- 

 zoic eras, all of which have had dramatic consequences 

 in the biologic history of the earth. 



Major periods of mountain building, the historical 

 geologist calls 'revolutions'; the lesser periods he calls 

 'disturbances' (the Rocky Mountains, Alps, and the Hi- 

 malaya are products of the relatively recent Laramide 

 and Cascadian 'disturbances'), and the intervening pe- 

 riods of quiescence he calls 'intervals.' During the inter- 

 vals the mountains raised in preceding orogenic episodes 

 have been largely if not entirely worn away by wind and 

 rain: it is estimated that, in the extreme, the total con- 

 tinental depth eroded and carried into the seas since the 

 opening of the Paleozoic era— that is, in only one-sixth 

 of the total history of the earth— would, if superimposed 

 in an unbroken vertical column, exceed ninety-five miles, 

 or thirty times the present Alps or Rockies. Through- 

 out most of earth's history, however, the general dis- 

 tribution of continents and oceans has been much the 

 same as it is today. 



Had the earth cooled with no mountain-building epi- 

 sodes, it would today be almost entirely covered by 

 oceanic waters from which there would protrude only 

 a few widely separated granite islands of relatively low 

 relief. Life would probably have been evolved in this 

 geographic wilderness of small islands in a imiversal sea, 

 but it is improbable that it would have taken the form 



