312 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



Let us take a little journey around the earth along the northern 

 zone. (Fig. 4.) Beginning at the Pacific coast, the land gradually 

 rises across California to the high plateau of Nevada and Utah, cul- 

 minating in Colorado. Thence it slopes gradually down, across the 

 Great Plains (Kansas), to the Mississippi Valley. Along this slope 

 practically no igneous rocks are met with, except for sporadic 

 and little-studied occurrences in the Ozark Ridge. The slight rise 

 seen in the Mississippi Basin is the level of the rocks of the Lake 

 Superior region (Minnesota, Wisconsin, Michigan). East of this 

 (Kentucky) few igneous rocks are known except those mentioned 

 above, and the land slopes gradually up to the Appalachian Ridge, 



Nevadi 

 California 



Lai. 40 -50° N 

 United States 



... .Colorado ,\$ «o°V-j \o' 



New England 



Ireland 

 England 



V France 

 Germany 

 'Alps 



flu stria 



Southern 

 Russia 

 Ural Mhx 



Pamirs. 

 Fig. 4. — Surface relief and specific volume. 



and east of this, across New England in the graph, descends to sea 

 level. 



The floor of the North Atlantic is rendered very summarily, and 

 the Azores and Iceland are about our only source of information as 

 to the composition of its floor along this zone. On the east coast of the 

 Atlantic the British Isles rise to but a small height (on the average) 

 above its surface. The average elevation of France is slightly higher; 

 that of Germany (which is inserted in the zone a little out of latitude) 

 is still somewhat higher, and thus we come to Switzerland and the 

 Tyrol, the culminating portion of Europe. To the east of this, with 

 an average elevation slightly greater than that of Germany, lies 



