Mkrriam. I 



John Day Basin. 



279 



The surface formation ever the greater part of the basin is the 

 Columbia lava, which has covered the whole region from the Cas- 

 cades to the eastern Blue Mountain ranges. At points where the 

 writer has been able to examine them, the existing northern and 

 southern ranges are capped with a considerable thickness of 

 Columbia lava. Ridges may, however, have existed in this region 

 as early at least as the beginning of the Miocene, since the John 

 Day deposits are either absent or much thinner along the existing 

 ranges. 



The Mascall beds are exposed principally in the southern part 

 of the basin and south of the Blue Mountains. They are usually 

 covered by a considerable thickness of Rattlesnake beds in which 

 a rhyolite flow has been the main protection against erosion. 

 The Rattlesnake beds are also confined, so far as the writer is 

 acquainted with them, to the southern region. Exposures of the 

 John Day beds and other pre-Columbia lava formations are found 

 only along the streams, where the lava has been cut through. 



PRE-CRETACEOUS FORMATIONS. 



The oldest fossil-bearing strata which have been found in the 

 John Day basin north of the southern Blue Mountain range are of 

 Cretaceous age. At several points in the basin formations have, 

 however, been found which have the appearance of being much 

 older than the Cretaceous beds. 



In the northeastern part of the basin a considerable area of 

 quartz diorite is exposed on the Middle Fork, about five miles 

 above Ritter. Where it was crossed by the writer, it is bordered 

 by sedimentary rocks which show a degree of induration and 

 deformation not met with in the Certaceous where it has been 

 seen in the basin. On Desolation Creek to the northeast of the 

 diorite area, this series seems to be extensively developed, and 

 would, doubtless, repay careful study. 



Opposite the Powell place, four or five miles above Ritter, on 

 Middle Fork, fossiliferous middle Tertiary beds rest upon the older 

 series. Columbia lava here covers the fossil beds and also laps 

 over upon the diorite, from which it is separated by only a thin bed 

 of ash. 



