The events and processes accompanying changes in the position of the southern margin of the North American continent during Jurassic and early Lower Cretaceous time comprise important chapters in paleogeo-graphic history. The shoreline during this interval extended across northern Mexico, from southern Coahuila to California, as shown by the similarity of faunas in these areas. Throughout Mesozoic time the sea advanced slowly northward, its progress being modified by orogeny and vulcanism. Evidence suggests that the region immediately to the south formed a negative block throughout Mesozoic, and possibly late Paleozoic, time, which permitted the sea periodically to cross central Mexico and separate two positive blocks. The solution of the problems presented by the ancient continental border has a direct bearing on the interpretation of the relations between the North American continental block, the Central American region, and the South American continental block. It relates the Mesozoic seaway across northern . . .