3°4 



University of California. 



[Vol. i. 



extent, forming probably the largest lava field of the world and 

 one of the most important formations on this continent. It would 

 seem advisable to restrict the name Columbia lava to this horizon. 



The lava series is composed of a large number of basalt flows 

 which are sometimes separated by beds of tuff. At Turtle Cove 

 twenty-three flows were counted in the bluff". Nowhere does it 

 seem to be less than i,ooo feet thick, and it would average much 

 above that over the greater part of the basin. 



Though no attempt was made to trace out any of the flows, it 

 was noted that some of them seem to extend for many miles. 



A hint as to the mode of exit of the lava is furnished by the 

 occurrence of basaltic dikes in the John Day beds at many locali- 

 ties. The largest and most important of those that have come to 

 the writer's notice are the Davis dikes, which run in nearly a 

 straight line, with few if any breaks, for about fifteen miles through 

 the lower end of Turtle Cove and out into the valley of the main 

 river. The dike shows transverse columnar structure at most 

 points. At the only place where its thickness was estimated, it 

 was about twelve feet in diameter, but it certainly reaches several 

 times that width along a good part of its course. 



In most places where the contact of dikes with the fossil beds 

 was observed, the bounding walls seem to have suffered little, if 

 any, change. At one point, however, on the west side of Turtle 

 Cove, where the Davis dikes cut the Upper John Day, a most 

 interesting contact phenomenon is noticeable. The ashy fossil 

 beds here exhibit a most remarkable development of columnar 

 structure for a distance of about fifteen feet from the lava on each 

 side. The columns are approximately normal to the surface of the 

 dike and fairly straight. The faces are very even, the columns 

 being as regular and sharply cut as the most perfect structures 

 developed in basalt. The dike at this point is multiple, and por- 

 tions of the fossil beds caught between the leaves have also 

 developed columnar structure, with this exception, that the 

 columns are there much larger than those bounding the dike on 

 the outside. The outer columns are noticeably small, averaging 

 about two inches in diameter, even when ten or more feet in 

 length ; while some of those developed in a band of John Day 



