16 EARTH FEATURES AND THEIR MEANING 



elevation and wrinkling of these deposits into marginal mountain 

 ranges. 



The continents and oceans which arose at the close of the 

 Paleozoic era. At the close of the second great era in the recorded 

 history of the earth, the now somewhat enlarged continents were 

 profoundly altered during a series of convulsive movements within 

 the surface shell of the lithosphere. When these convulsions were 

 over, there was a new disposition of land and sea, but one quite 

 different from the present arrangement. Instead of being ex- 

 tended in north-south belts, as they are at present, the continents 

 stretched out in broad east-west zones, one in the northern and 

 the other in the southern hemisphere. To the broad southern 

 continent of which so little now remains, the name " Gondwana 

 Land " has been given, and to the sea which divided the northern 

 from the southern continent the name " Ocean of Tethys." The 

 northern continent stretched across the site of the present Atlantic 

 Ocean as the " North Atlantis/ 7 its northern shore to the west- 

 ward being somewhat farther south than the present northern 

 coast of North America, since life forms migrated in the north- 

 ern ocean from the site of Behring Sea to that of the present 

 North Atlantic. 



This arrangement of land and water during the middle period 

 of the earth's recorded history, when considered with reference 

 both to its earlier and to its later evolution, may perhaps be best 

 accounted for by the assumption that the lithosphere was then 

 shaped like Fig. 5 (middle). In this figure two truncated tetra- 

 hedrons are joined in a common plane of contact which may be 

 described as the twin plane. This medial depression upon the 

 lithosphere was occupied by the intercontinental sea, the Ocean of 

 Tethys. 



Near the close of this second great era of the earth's conti- 

 nental history, crustal convulsions, which were perhaps the most 

 remarkable in the entire record, resulted in the almost complete 

 disappearance of the southern continent and a concentration of 

 the land within the northern hemisphere as a somewhat inter- 

 rupted belt surrounding a central polar ocean (Figs. 3 and 5). 



Upon the assumption of twin tetrahedrons in the intermediate 

 era of continental evolution, both the Ocean of Tethys of that 

 time and its present remnants, the Caribbean and Mediterranean 



