Graptolite-rich detrital Ordovician rocks of siliceous, argillaceous, and volcanic composition are exposed in scattered areas in westernmost North America from Convict Lake in the Sierra Nevada, through Nevada, central Idaho, British Columbia, and Yukon Territory to Alaska. The paleogeographic relation of this facies to that of carbonate rocks, rich in shelly faunas, is not everywhere clear. In Alaska large areas of Ordovician eugeosynclinal rocks may lie unrecognized. Though graptolites are disappointing as paleoecologic indices in the eugeosyclinal strata, their presumed planktonic or epiplanktonic habit should be useful in reconstruction of former ocean currents and the lands between which they flowed.

You do not currently have access to this article.