16 p. B. KING 



deformation, but most, in a sense, are endless. Those along the edges 

 of the Atlantic Ocean run out to sea and apparently are broken off at 

 the edges of the continental shelves so that their further extensions 

 are lost. Those around the margins of the Pacific Ocean, however, 

 such as the belt of the North American Cordillera, are continuous or 

 nearly so from one continent to the next, and they appear to be 

 parts of a single great zone of deformation. In the circum-Pacific 

 zone, orogeny created potential conditions for land bridges between 

 the continents, and they came into being from time to time during 

 the orogenic and post-orogenic phases. Such land bridges were along 

 the present and observable orogenic trends; as already noted, no 

 land bridges could have formed across the ocean basins. 



The record suggests that land connections along the circum.- 

 Pacific orogenic belt were firmer and more frequent between North 

 America and Asia than between North America and South America. 

 Much of the crust beneath the seas of Middle America, as in the Gulf 

 of Mexico and Caribbean Sea, is more oceanic than land-laid and 

 seems to be in process of transformation into land-laid crust by 

 sedimentation on its surface, and by magmatic transformation 

 within it (Ewing et al., 1957, pp. 909-911). At least a part of the firm 

 land connection between the two continents in the Central American 

 isthmus, especially in Nicaragua and Costa Rica, was built up rather 

 recently by volcanic eruptions: "To be sure it is in fact an isthmian 

 link. But why is it such an outrageous isthmian link?" (Woodring, 

 1954, p. 730). Before development of the isthmus, the lands along 

 the orogenic belt between North and South America were mainly 

 disconnected islands. 



Deformation of Eugeosynclinal Belt of Cordillera 



Cordilleran eugeosynclinal rocks were deformed, metamorphosed, 

 and invaded by granitic rocks in the middle and last half of the 

 Mesozoic era, during a time referred to as the Nevadan orogeny (Fig. 



4)- 



On the west slope of the Sierra Nevada, for which the orogeny is 



named, Jurassic rocks are steeply upturned, altered, and invaded by 



plutonic rocks, whereas Cretaceous rocks lie on their deeply eroded 



edges with little disturbance. From this relation and from more 



detailed evidence we need not mention here, some geologists have 



concluded that at least the deformational phase of the orogeny in the 



