THE FIGURE OF THE EARTH 



15 



record came at the end of the first great era of geologic history, 

 the so-called Eozoic era; a second great transformation came at 

 the close of the second or Paleozoic era ; and a third began at the 

 end of the next or Mesozoic era, an adjustment which is apparently 

 continuing to-day. Each of these great surface deformations was 

 accompanied by great volcanic eruptions of which we have the 

 evidence in the lavas remaining for our inspection, and each was 

 followed by the formation of great glaciers which spread over 

 large areas of the existing continents. 



Before the earliest of these great changes, the earth appears to 

 have approximated in its figure somewhat closely to the ideal 

 spheroid, for it was everywhere enveloped in the hydrosphere as a 

 universal ocean. Toward the close of this period came the adjust- 

 ments which brought the lithosphere to protrude through the 

 hydrosphere in shield-like continents whose distribution, as shown 

 by the rocks of this period, is of great significance. Within the 

 northern hemisphere rose three land shields spaced at nearly 

 equal intervals and at nearly equal distances from the northern 

 pole. One of these was centered where now is Hudson Bay, 

 another about the present Baltic Sea, and the relics of the third 

 are found in northeastern Siberia. These earliest continents 

 have been referred to as the Laurentian, Baltic, and Angara shields. 

 Within the southern hemisphere shields appear to have developed 



Ar eno of eozotc En* 



/vo of PSU.COZO/C /?, 



FIG. 5. Approximations to earlier and present figures of the earth. 



in somewhat similar grouping, namely, in South America, in South 

 Africa, and in Australia (Figs. 3 and 5). 



These coigns or angles of a form into which the earlier spheroid 

 of the earth was being transformed have persisted through the 

 greater part of subsequent geologic time, and have been enlarged 

 by the growth of sediments about them as well as by the later 



