2 PROCBEDI]SrGS OF THE NATIONAL MUSEUM vol.73 



northwest in narrow valleys that lie from 1,000 to 1,500 feet below 

 the adjacent rolling uplands. Locally the main roads follow the river 

 valleys, but travel northward is difficult. If the spring were more 

 accessible, it could form the basis of a popular resort. 



Pre-Tertiary rocks. — The stratified rocks exposed at the surface 

 in the region are Carboniferous and Triassic and include limestones, 

 argillites, and quartzites. These are intruded successively by gabbro, 

 peridotite (now largely altered to serpentine), and several varie- 

 ties of granite, all of the pre-Cretaceous age.^ During late Mesozoic 

 time these rocks were extensively eroded, so that the region west 

 of Blue Mountains was reduced to low relief. Doubtless some of 

 these rocks underlie the area near Kitter Hot Spring at no more 

 than 1,000 feet below the surface. 



Tertiary rocks. — The rocks of Tertiary age in the region include 

 a wide range of volcanic tuffs and flows separable on lithologic and 

 structural bases into five formations (ascending order) : Clarno and 

 John Day formations, Columbia River basalt, and Mascall and 

 Rattlesnake formations.- Ten miles east of Ritter the Columbia 

 River basalt rests on pre-Cretaceous altered gabbro, but 30 miles 

 west the other Tertiary beds attain a maximum thickness of 5,000 

 feet. Only the Columbia River basalt flows and associated scoria 

 outcrop near Ritter, and probably none of the earlier Tertiary rocks 

 separate the basalt from the underlying pre-Cretaceous rocks. 



The Columbia River basalt is well exposed on the walls of the 

 Canyon of the Middle Fork of John Day River, which is 1,300 feet 

 deep at Ritter. This thickness is made up of many flows, each of 

 which displays a dense columnar phase overlain by vesicular phase 

 and more or less scoria. The columnar phase is dark greenish black, 

 but the vesicular phase and scoria are mingled dark green and brick 

 red. Most of the flows range from 50 to 125 feet in thickness. In 

 the small area shown in Figure 1 only three flows are exposed, a par- 

 tial section of which is given below : 



Section of ColumMa River basalt at Ritter Eat Spring, Oreg. 



Upper flow, vesicular and scoria phases. Thickness 



Upper flow, columnar phase feet__ 15-20 



Middle flow, vesicular phase do 30-40 



Middle flow, columnar phase do 15-25 



Lower flow, vesicular and scoria phases do 50 



Lower flow, columnar phase. 



1 Lindgren, W., The Gold Belt of the Blue MouDtams of Oregon : Twenty-second Ann. 

 Rept. U. S. Gcol. Survey, pt. 2, pp. 561-776, 1901. 



Pardee, J. T., and Hewett, D. P., Geology and mineral resources of the Sumpter quad- 

 rangle : Oregon Bur. of Mines and Geology, vol. 1, No. 6, pp. 7-128, 1914. 



« Merriam, J. C, A contribution to the geology of the John Day Basin : Univ. of Cali- 

 fornia, Dept. of Geol., Bull., vol. 2, pp. 269-314, 1901. 



Collier, A. J., Geology and mineral resources of the John Day Basin : Oregon Bur. of 

 Mines and Geology, vol. 1, No. 3, pp. 1-47, 1914. 



