CLOSE OF THE PERIOD 459 



em coast of the Great Basin land. These strata have yielded 

 many marine fossils, but the fossils are very scanty as compared 

 with those of the beds formed in the open sea. 



In the later part of the Jura, or upper Malm, still further changes 

 were produced. In the Old World this was the time of a vast 

 transgression of the sea, but in North America the land was rising, 

 gradually drying up the great northern gulf, interrupting the con- 

 nection with central European waters which had so long been 

 shown by the fossils of the Pacific border, and bringing in a 

 transgression of the Arctic Sea, which extended its waters all down 

 the western coast of North America as far as Mexico, but was sep- 

 arated in some way from the sea which washed the west coast of 

 South America. Upper Jurassic strata are found in British Colum- 

 bia and California, where they form an enormously thick series, 

 principally of slates, with interstratifled beds of diabase tuffs, which 

 show that volcanic activity prevailed along the shores, or in the 

 bed of the sea. In the Sierra Nevada these slates are traversed 

 by numerous gold-bearing quartz veins. 



The close of the Jurassic was accompanied in North America 

 by a time of upheaval and mountain making along the western 

 side of the continent, corresponding to the Appalachian revolution 

 which had occurred at the close of the Palaeozoic era along the 

 eastern side. The Sierra Nevada had for long ages been a sinking 

 geosynclinal trough, in which great thicknesses of sediment had 

 accumulated ; now, at length, it yielded to the force of lateral 

 compression, which ridged the strata up into great folds. By this 

 movement the Pacific coast-line was transferred from the eastern to 

 the western side of the mountains, and probably the Coast Range 

 was upheaved, forming a chain of islands. Little is known con- 

 cerning this primary condition of the Sierra Nevada, which had 

 not yet become separated from the Great Basin by faults, the 

 present mountains being due to long subsequent movements. In 

 the interior of the continent also these movements brought about 

 great changes, though it is not probable that the geographical modi- 

 fications of the interior all occurred at the same time, or that they 

 were all connected with the upheaval of the Sierra Nevada. We 



