PALEOZOIC CORDILLERAN GEOSYNCLINE 



to their source, despite the fact that their size and abundance indicated 

 to him a nearby local origin. It seems necessary, he believes, to assume 

 that granitoid intrusions existed in a land that formerly stood to the 

 west where only the Pacific Ocean now lies. 



Krynine (1941) has studied the tectonic significance of arkoses, and 

 concludes that they are deposited when a granitoid terrane has just been 

 uplifted and is being vigorously dissected. They are related to the de- 

 formed geosyncline into which granitoid rocks have been intruded. The 

 plutons have become exposed by erosion of the mountains created by 

 the orogeny, and then uplifted in a further stage of deformation, and 

 vigorously eroded. 



Granite plutons are seldom exposed in arcs of small volcanic islands. 

 We must look to the larger islands of an archipelago for the source of 

 granitoid conglomerates and arkoses. The geologic map of the Japanese 

 Archipelago, Fig. 6.20, shows extensive areas of granitic intrusions and 

 Precambrian gneisses which could furnish the necessary material. The 

 major archipelago like the Japanese has had a long orogenic history and 

 is composed not only of rocks that will make graywackes but also arkoses. 

 Such a one seems to have been the sourceland of the sediments of the 



■ western part of the Cordilleran geosyncline. 



Great beds of chert are present in the sediments of the volcanic archi- 

 pelago. Extensive beds of chert and cherty limestone are present in the 



1 miogeosyncline as well as in the inland basins and shelfs of the main- 

 land, and so the factors governing the precipitation of the silica are 



j probably several. Its transportation in solution in marine currents may re- 



1 suit in precipitation a great distance from its source. I find it easy to be- 

 lieve that a large part of the silica originated in the volcanic activity of 

 the archipelago, that some of it was carried by currents across the seas 

 between the archipelago and the mainland free from the area of deposi- 



I Fig. 6.20. Generalized geologic map of the Japanese Archipelago and the easten part of Asia. 

 Isobaths are in meters. Coarsely stippled areas are those chiefly of sedimentary rocks but with 

 large areas of Archean gneiss and schist and some smaller areas of intrusive and extrusive rock. 



; Finely stippled areas denote alluvium. Hachured areas are those of plutonic rocks, chiefly granite 

 and granadiorite, but with considerable areas of Archean gneiss and schist and some sedimentary 

 rocks. Solid black areas are andesite. Horizontally ruled is basalt and verticlly ruled is trachyte. 



300 MILES 



