MEKR 1AM. I 



John Day Basin. 



in it. The columns are here about 12-15 inches in diameter. 

 They are sharply separated from each other and show remarkable 

 regularity. The rhyolite extends as a very uniform flow for at 

 least fifty miles westward from Mt. Vernon. It is said by Mr. 

 Osmont to reach up the valley at least 1 5 miles above Canon City. 

 Osmont also states that it caps the Mascall formation in the 

 Crooked River region. Being much more resistant than the 

 underlying beds, it has tended to form broad tables or terraces 

 wherever it occurs. 



Fossils. — The Rattlesnake gravels contain many vertebrate 

 remains, most of which have heretofore been listed with the 

 Mascall fauna. The Rattlesnake fossils when weathered out are 

 frequently to be found resting upon the Mascall beds below, and 

 as most of the material from both Rattlesnake and Mascall is found 

 detached from the matrix, the difficulties in the way of separating 

 the faunas are considerable. So far no segregated lists of the 

 species have been published. 



Basin of Deposition. — The floor upon which the Rattlesnake 

 was laid down consisted largely of the upturned and eroded Mas- 

 call beds. The basin of deposition was bounded on the north 

 by a range of hills formed by the upper portion of the Columbia 

 lava monocline. At the present time the northern limits of the 

 basin may be clearly seen, as large portions of the deposit have 

 escaped erosion and extend along the northern side of the East 

 Fork valley and westward to the divide some miles beyond Caleb. 

 These considerable remnants all fall into a nearly horizontal plane 

 which abuts against the lava hills to the north. Looking out over 

 the Rattlesnake table-lands from Spanish Gulch one can see an 

 almost continuous series of these deposits, which are in nearly 

 their original position and have not suffered greatly from erosion. 

 It requires no great effort for one to picture to one's self this part of 

 the basin in its original form. 



Deformation. — At Picture Gorge, on the north side of the valley, 

 the Rattlesnake rhyolite extends to within a few yards of the 

 Columbia lava and would rest upon it if the section were not 

 interrupted by the gorge. On the south side of the valley it is not 

 well represented, and, where present, is frequently much broken up. 



