294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 3 



have pulled the rocks apart, allowing great wedge-shaped blocks to 

 sink into troughlike areas, as that of the Great Basin, the Rhine 

 Valley, and our ow^n Triassic structural basin between Chapel Hill 

 and Ealeigh, By the use of the pendulum man is able to measure 

 the relative mass of the various earth segments. It is found that 

 the mountain masses are made of material lighter than the average 

 of the earth's crust, and observations show that wherever the earth 

 is receiving a load it tends to sink, as at present in the Mississippi 

 Delta region, or in the northern United States during the ice inva- 

 sion; also where load is being removed the earth's surface rises, as 

 in many mountain areas losing weight by erosion, or as in the 

 northern United States after the retreat of the great ice sheet. These 

 compensating movements are very slow and have a considerable lag. 

 Following the retreat of the ice sheet the Lake Champlain region, 

 for example, was buried at first several hundred feet beneath sea 

 water. Compensating forces (isostasy) have but recently finished 

 returning this area to its normal elevation. 

 A study of the earth's mountains shows : 



1. Mountains have generally been formed along the border of the 

 land mass which furnished the sediments now found in them. (The 

 sediments in the Appalachian Mountains came from a great land 

 mass, Appalachia, now occupied by the combined Piedmont, Atlan- 

 tic Coastal Plain, and the Continental Shelf.) 



2. Young mountains are usually nearer the present border of the 

 continent than the older mountains. (In western North America 

 the Coast Ranges are younger than the Sierras or the Rockies.) 



3. All older mountains, which have been deeply eroded, show a 

 core of igneous or metamorphic rock. 



4. Youthful, growing mountains are usually earthquake and vol- 

 canic zones, as witness the borderlands of the Pacific and Mediter- 

 ranean areas. 



5. Growing coastal mountains usually have great ocean deeps in 

 front of them, the so-called " ocean foredeeps ", as, for example, the 

 six different foredeeps west of the Andes, the Supan foredeep of the 

 Aleutian Islands, and the Tuscarora foredeep of the young moun- 

 tain range of the Japanese Islands. 



6. Mountain-making is going on seemingly with as great force as 

 ever in the earth's history. All about the Pacific, lands are being 

 elevated, and in many places the ocean foredeeps are sinking, fre- 

 quently with sudden slips and violent earthquakes. 



7. Mountain-making, accompanied by great continental emergence, 

 has brought to a close each of the great geologic eras. 



8. Many of the great mountains are curved or arcuate in character. 

 All the present-day growing mountain-arcs, as the Aleutian Arc, the 



