28 .  . DYNAMICAL GEOLOGY. [Boor Til. | 
distinct bedding along the Snake River ravines. It arose from ~ 
what may have been on the whole a continuous though locally 
intermittent welling out of lava, probably from many fissures ex- 
tending over a wide tract of Western America during a late Tertiary 
period, if, indeed, the eruptions did not partly come within the time 
of the human occupation of the continent. 
At a few points on the plain and on its northern margin the author 
observed some small cinder cones (Fig. 63). These were evidently 
formed during the closing stages of volcanic action, and may be 
compared to the minor cones on a modern volcano, or better, to those 
on the surface of a recent lava-stream.. 
In Europe during Miocene times similar enormous outpourings of 
basalt covered many hundreds of square miles. The most important 
of these is that which occupies a large part of the north-east of — 
Treland, and in disconnected areas extends through the Inner 
Hebrides and the Farde Islands into Iceland. Throughout that 
region the paucity of evidence of volcanic vents is truly remark- 
able. So extensive has been the denudation that the inner structure 
of the volcanic plateaux has been admirably revealed. The 
ground beneath and around the basalt sheets has been rent into 
innumerable fissures which have been filled by the rise of basalt into 
them. A vast number of basalt-dykes ranges from the volcanic 
area eastwards across Scotland and the north of England. Towards 
the west the molten rock reached the surface and was poured out 
there, while to the eastward it does not appear to have overflowed, or 
at least, all evidence of the out-flow has been removed in denudation. 
When we reflect that this system of dykes can be traced from the 
Orkney Islands southwards into Yorkshire and across Britain from ~ 
sea to sea, over a total area of probably not less than 100,000 square — 
miles, we can in some measure appreciate the volume of molten 
basalt which in Miocene times underlay large tracts of the site of 
the British Islands, rose up in so many thousands of fissures, and 
poured forth at the surface over so wide an area in the north-west. 
In Africa vast basaltic plateaux occur in Abyssinia, where by 
the denuding effect of heavy rains they have been carved into 
picturesque hills, valleys, and ravines.t In India an area of at least 
200,000 square miles is covered by the singularly horizontal volcanic 
plateaux of the “Deccan Traps” (lavas and tuffs), which belong to 
the Cretaceous period and attain a thickness of 6000 feet or more.? 
The underlying platform of older rock, where it emerges from 
beneath the edges of the basalt tableland, is found to be in many 
places traversed by dykes; but no cones and craters are any where 
visible. In these, and probably in many other examples still un- 
described, the formation of great plains or plateaux of level sheets 
of lava is to be explained by “fissure-eruptions” rather than by the 
operations of volcanoes of the familiar “ cone and crater” type. 
! Blanford’s Abyssinia, 1870, p. 181. 
* Geology of India, Medlicott and Blanford, p. 299. 

