THE OROGENIC EPOCHS IN NORTH AMERICA 647 



rocks of this age are comparatively scarce in the plateau region be- 

 tween the Pacific and Rocky Mountain systems. Such facts as are 

 now available suggest that the western limit of the Laramide dis- 

 turbance passes through central Arizona, eastern Nevada, western 

 Idaho, and central British Columbia to the Yukon valley. This 

 tentative division of the provinces is supported by the fact that the 

 Comanchean rocks in the Bisbee region, Arizona, have been gently 

 tilted and faulted but only slightly folded ; the Cretaceous rocks near 

 the headwaters of the Yukon River have been folded, and the same is 

 true of beds of similar age on the northeast side of the Seward 

 Peninsula in Alaska. On the other hand, along the Pacific slope in 

 California, Oregon, Washington, British Columbia, and Alaska, 

 wherever both upper Cretaceous and Eocene strata are present, 

 they are generally either conformable, or separated by a mere dis- 

 conformity. At the north end of the Sacramento Valley, California, 

 the upper Cretaceous strata still remain almost horizontal. 



There is substantial agreement among geologists that the 

 Laramide orogeny marks approximately the close of the Cretaceous 

 period. Nevertheless, until the existing controversy over the cor- 

 relation of the latest Cretaceous and earliest Eocene formations of 

 western United States is satisfactorily settled, the exact date of 

 the deformation remains uncertain. In some parts of western 

 Wyoming and Colorado there are suggestions of two episodes of 

 crumpling separated by a short but definite epoch of erosion and 

 the deposition of sediments in the earliest Eocene (Paleocene). 

 Similar phenomena are reported from Yellowstone Park and 

 southwestern Montana. 



Throughout its range the Laramide orogeny was marked by 

 great volcanic activity, which manifested itself in the production of 

 laccoliths and volcanic cones with their associated dikes, sills, 

 flows, and fragmental deposits. Locally in parts of Idaho and 

 western Montana granitic batholiths were intruded at this time, 

 but they are much less characteristic of the Laramide system than 

 of the Nevadian. 



Antillean orogeny {middle-to-late Miocene). — It has long been 

 known that the dominant structural features of the California and 

 Oregon coast ranges were produced in the Miocene period. To 



