8 



p. B. KING 



average compositions of the crust beneath the two areas (Ewing and 

 Press, 1955). Crust of the ocean basins is made up of relatively 

 dense rock, called sima, with an average composition about like 

 basalt, and with a thickness beneath the ocean water of about 6 

 miles. Simatic material like that beneath the oceans also forms the 

 base of the thicker crust of the continental platforms, but most of 

 the thickness of the platforms consists of lighter rock, called sial, 

 of about the composition of granite. Continental crust has a thick- 

 ness on the order of 20 miles. 



Table I. Subdivisions of Later Geologic Time, and Their Relation to 

 the Phases of the Evolution of the Cordilleran Region 



These differences in composition and thickness of the crust under- 

 score a widely held belief in the permanence of continents and ocean 

 basins, and give little comfort to notions of vanished lands within 

 the ocean areas. To create and destroy such lands would involve not 

 merely changes in level, but changes in crustal composition. It is 

 agreed that the Pacific Ocean basin, in particular, was a permanent 

 crustal and topographic feature throughout known geologic time. 

 Even the relatively modest ''borderlands" that some geologists have 

 believed once existed along or off the present coasts must each be 



