110 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



position of the volcanic funnel has been largely cut away by the waves, and is almost 

 entirely isolated among them. But the inarch of destruction has been greater in Skye. 

 The connection between the vent and the materials ejected from it has been entirely 

 removed, and we can only guess from the size of the remaining neck what may have been 

 the area covered by the discharges from this largest of all the volcanic cones of the Inner 

 Hebrides. 



In bringing this part of my subject to a close, I would repeat that the distinctive 

 characters of the basalt-plateaux lead us to seek the modern analogies to these volcanic 

 phenomena, not in large central cones discharging streams of lava in different directions 

 like Vesuvius or Etna, but in those basalt-regions where the lavas have issued from 

 innumerable minor and sometimes almost imperceptible vents.* I have already referred 

 to a journey made by me in 1879 through some of the vast basalt-fields of Western 

 America, and to the light which I thereby gained on the history of the youngest volcanic 

 tracts of Britain. The basalt of Idaho stretches out as a vast and apparently limitless 

 plain. Along its northern boundary, this sea of black lava runs up the valleys and round 

 the promontories of the older trachytic hills, with almost the flatness of a sheet of water. 

 It has been deeply trenched, however, by the streams that wind across it, and especially 

 by the Snake River, which has cut out a gorge some 700 feet deep, on the walls of which 

 the successive beds of basalt lie horizontally one upon another, winding along the curving 

 face of the precipice exactly as those of Antrim and the Inner Hebrides do along their 

 sea- worn escarpments. Here and there, a low cinder-cone on the surface of the plain 

 marks the site of a late outflow. One is struck, also, with the singular absence of tuffs 

 and volcanic conglomerates. The basalts appear to have flowed out stream after stream 

 with few fragmentary discharges. 



These characteristic features of one distinctive type of volcanic action have been 

 repeated over a vast region, or rather a whole series of regions, in Western America, the 

 united area of which must equal that of a considerable part of Europe. From Idaho, the 

 basalt-fields may be followed southwards interruptedly into Utah and Nevada, and across 

 the great plateau-country of the canons into Arizona and New Mexico, northwards into 

 Montana, and westwards into Oregon. The tract which has as yet been most carefully 

 traversed and described is probably that of the high plateaux of Utah and Arizona. 

 Thus on the Uinkaret plateau, which measures some 45 to 50 miles in length by 8 to 12 

 in breadth, a thick sheet of basalt has been spread consisting of many successive flows. 



* In this connection I may again refer to Hopkins's Researches in Physical Geology, where the conditions of the prob- 

 lem here discussed have been distinctly realised. Speaking of the ejection of lava from a number of fissures, he remarks 

 that the imperfect fluidity of the melted material " would seem to require a number of points or lines of ejection as a 

 necessary condition." " If there were only a single centre of eruption, a bed of such matter approximating to uniformity 

 of thickness, could only be produced on a surface of a conical form." " Where no such tendency to this conical structure 

 can be traced, it would probably be in vain to look for any single centre of eruption. On the supposition, too, of 

 ejection through continued fissures, or from a number of points, that minor unevenness of surface which must probably 

 have existed under all circumstances during the formation of the earth's crust, would not necessarily destroy the 

 continuity of a comparatively thin extensive bed of the ejected matter, in the same degree in which it would inevitably 

 produce that effect in the case of central ejection " (Cambridge P\il. Trans., vi. (1835), p. 71). 



