90 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



tion of volcanic material, and that it was precipitated copiously in the 

 shallow seas of the eastern trough and mainland shelf where, from place 

 to place and time to time, clay, lime mud, or sand were accumulating. 

 Perhaps the tectonic conditions of the eugeosyncline of the western 

 margin of North America can be visualized better if reference is made to 

 the Japanese Archipelago (see Fig. 6.20). It is believed that there a 

 fairly good example and close parallel of conditions exists now as ex- 

 isted in times past along the west coast of North America. 



In the first place the scale and shape of the arcuate features are the 

 same. In the second place, the geology of the Japanese Archipelago is 

 somewhat the same as that postulated for the sourceland of the sediments 

 of the Pacific trough of the Paleozoic Cordilleran geosyncline. The most 

 abundant rocks mapped in the Japanese Archipelago are as follows: 

 andesite, granite, syenite, schistose granite, gneiss, schist, slate, chert, 

 sandstone, limestone, diorite, pyroxenite, amphibolite, gabbro, and 

 trachyte, in approximate descending order of abundance. 



