CHAP. XL 
THE MODERN ERUPTIONS OF ICELAND 
267 
acid eruptions are known to have occurred in modern times, some of the 
liparites are crusted with sulphur and from the connected fissures steam 
still rises. . , . » 
It will thus be seen how entirely the modern volcanic eruptions 0 
Iceland agree with the phenomena presented by our Tertiary basalt- 
plateaux. It is, therefore, to the Icelandic type of fissure-eruptions, and 
not to great central composite cones like Vesuvius or Etna that we 
must look for the modern analogies that will best serve as continental} 
and explanation for the latest chapter in the long volcanic history of the 
British Isles. 1 „ . , . 
As a further but more ancient illustration of the type of volcanic action 
which appears to have been prevalent during the formation of the Tertiary 
volcanic plateaux of Britain, I may again refer to the vast basalt-fields ol 
Western America. The basalt of Idaho stretches out as an 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, how- 
ever, 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 ol 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, however, with the singular absence of tuffs and volcanic con- 
glomerates. 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 ol regions, 111 
Western America, the 'united area of which must equal that ol a consider- 
able part of Europe. From Idaho, the basalt-fields may be followed south- 
wards 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 covering of basalt 
has been spread composed of many successive flows. Between 100 and , 0 
separate cones have been counted on this area, most of them quite sma , 
mere low mounds of scoriie, though a few reach a height of 700 or 800 feet, 
with a diameter of a mile. From three to seven or eight may be found 111 
a row, as if springing from a single line of fissure. But generally the 
1 Iu his memoir of 1874, Professor Judd announced Ids conclusion that there were formerly 
live great volcanoes amongst the Western Isles, and that the lavas of the plateaux had issued 
from these. He. subsequently reiterated this view (Quart. Jmm. Geol. !>oc. xlv., 1890, p. 1 ), 
and ridiculed the explanation of fissure-eruptions The evidence adduced byme n me. 
published in 1896 (same journal, vol. lii. p. 331) and reprinted with additions in this Chapter, 
will, I trust, be regarded by geologists as having finally settled this question. 
