238 Proceedings of the Royal Physical Society. 



"neck" or pipe can be referred to the Tertiary period. 

 Among the terraced basalt plateaux of Antrim and the Inner 

 Hebrides one finds the same want of evidence of cones and 

 craters from which these wide sheets of lava proceeded.* 

 Mile after mile the beds of basalt may be followed in nearly 

 horizontal position with wonderful persistence and unifor- 

 mity of thickness. Very rarely, too, are they separated from 

 each other by beds of tuff. Unlike the mingled sheets of 

 lava and ashes constituting the materials ejected from such 

 cones as Vesuvius or Etna, the Miocene basalts form nearly 

 the whole of their great plateaux, with here and there thin 

 bands of clay or coal full of the remains of the terrestrial 

 vegetation covering the land which they overflowed. I am 

 persuaded that the explanation of the origin of these widely 

 extended sheets of basalt is to be sought not in the familiar 

 or Vesuvius type of a volcano, but in those great fields of lava 

 which cover such wide tracts in the Western States and terri- 

 tories of North America and in the Deccan. According to 

 Whitney, the area covered by lavas between the Sierra Nevada 

 and the Eocky Mountains fully equals the whole extent of 

 France. As has been well shown by that geologist, by Kicht- 

 hofen, King, Hayden, and many other observers, these basaltic 

 floods belong to the closing epochs of volcanic activity, after 

 a long succession of earlier rhyolitic and trachytic eruptions. 

 In a recent visit to the basalt plain of the Snake Eiver, 

 in Idaho, I was greatly impressed by the analogies presented 

 by it to some of the more notable features of our Tertiary 

 basalt plateaux. The basalt of that arid plain has flowed 

 along the base of the hills, running in and out exactly as a 

 lake of water does round the bays and promontories of its 

 margin. Many of these hills are formed of trachyte which 

 had been much eroded into hollows before the eruption of the 

 basalt. The great Canon of the Snake Eiver has been exca- 

 vated through the cake of basalt and partly in the under- 



* Mr Judd {Quart. Jour. Geol. Soc, vol. xxxii,, p. 292) has described 

 what he believes to have been some of the miocene vents of the West High- 

 lands ; but even on this view they are altogether inadequate to have supplied 

 the immense volume of lava, while they leave unexplained the persistence, 

 horizontality, and regularity of the basalt sheets and the absence of tufl'is. 



