AND ITS INHABITANTS 71 



as the basis for dividing the eras into periods of time, and how 

 often they occurred is one of the things that geologists are 

 trying to ascertain. In North America we know of at least 

 eight of these minor crustal readjustments, but in all the world 

 there are many more than this. 



Finally, there comes a time of major shrinking that adjusts 

 all of the strains and stresses set up in the earth's mass by the 

 minor, incompletely adjusted shrinkages. The earth has just 

 passed through one of these major readjustments, and accord- 

 ingly we see all of the continents standing far higher above 

 sea-level than has been the rule throughout geologic time, and 

 in many of them rise majestic ranges of mountains (see 

 Fig. 3). A grander, more diversified, and more beautiful 

 geography than the present one the earth has never had; this 

 statement is made advisedly and with the knowledge that our 

 planet has undergone at least six of these major readjustments 

 of its mass.' l l'tese greater movements are the "revolutions" 

 that close the eras. Because the lands are then high they are 

 subject to more active erosion and in the last analysis all of 

 the broken-up detrital and dissolved material is carried away 

 by the streams to the oceans. At these times the continents are 

 also largest and the materials received by the oceans are laid 

 down on the outermost edges of the lands, where subsequent 

 transgressions by the sea cover and hide them from our 

 observation. After a long time the sea again comes to press 

 further and further upon the land and spreads more forma- 

 tions of stratified rocks over those left by the previous flood- 

 ings, the older geologic formations (see Fig. n). Therefore 

 there is upon the present continents between each two such 

 successive formations a "break" in deposition, a hiatus in the 

 geologic record, a "time interval" when no record other tha 

 erosion is at hand. These "breaks" in sedimentation are 

 representative of loss of record and are called "intervals"; 

 they are regarded as the closing times of the eras. 



