776 J I. 8. WASHINGTON DECCAN SALTS AND PLATEAU BASALTS 



region because the name "Oregon" was originally given, so far back as 

 1778; to the river now called the Columbia, and Avas afterward applied, 

 somewhat vaguely, to the country drained by this river. The name was 

 preserved for many years in that of the old Oregon Trail. 12 



The basaltic flows which occupy the two river valleys and the surround- 

 ing country belong obviously to one comagmatic region, and may there- 

 fore be discussed together. Parts of the region have been described more 

 or less by many writers, of whom only a few may be cited here. 13 The 

 region as a whole, however, has never been adequately treated and a 

 monographic description of the whole region is much to be desired. 

 Nearly all those who describe these flows remark on their similarity to 

 the Deccan traps. 



The area covered at present by the Oregonian basalts is at least 200,000 

 or 250,000 square miles, approximately that of the Deccan traps 14 . The 

 maximum thickness of the series of flows is about 4,000 feet, and 

 Campbell calculates the total volume as 24,000 cubic miles, assuming 

 an average thickness of only 500 feet. The number and thickness of 

 the separate flows differ at different localities, and the average thickness 

 appears not to have been estimated. The basalts were considered by 

 Geikie to have issued from fissures, and Russell, Campbell, and practi- 

 cally all others concur in this view. Some low, broad cones, made up 

 of ash or flows or both, occur throughout the area, especially on the 

 Snake River plateau, but they are inconspicuous and constitute a minor 

 feature. Some ash beds and layers of soil are intercalated between the 

 beds in places, and volcanic bombs are fairly common. Pre-Tertiary 

 rhyolites occur with the basalts, but the relation between these rhyolites 

 and the basalts is not clear, and no analyses of these rhyolites seem to 

 have been published. The great extension and substantial horizontality 

 of the Oregonian basalts and the striking way in which, water-like, they 

 flowed over and submerged the preexisting topographic features, indi- 

 cating a high degree of fluidity, are the chief physical characters of the 

 region, in which it is at one with that of the Deccan. 



The age of the earlier of the Oregonian flows is assigned to the Eocene, 

 and the flows seem to have reached their maximum in the Miocene; but 

 Russell thinks that flows "occurred probably within recent historical 

 times and are, perhaps, not over 100 to 150 years old." The flows of the 



12 Cf. F. W. Hodge : Handbook of American Indians. Bureau American Ethnology. 

 Bull. 30. vol. ii, 1912, p. 146 ; J. S. Diller : U. S. Geol. Survey, Bull. (514. 1915. p. 28. 



13 1. C. Russell : Volcanoes of North America. New York, 1897, pp. 250-257 ; Russell : 

 U. S. Geol. Survey, Bull. 199, 1902 ; G. O. Smith : U. S. Geol. Survey, Folio 106, 1904 ; 

 M. R. Campbell ct al. : TJ. S. Geol. Survey, Bull. 611, 1015, pp. 165-168. 



14 A sketch map is given by Campbell, op. cit., sheet 23. 



