It has frequently been asserted that in the great districts covered by basaltic lavas which we find in the Rocky Mountains of North America, in the Deccan of India, in Abyssinia, and even in the Western Isles of Scotland, we have proofs of the occurrence, during earlier geological periods, of volcanic action very different in character from that which at present takes place on our globe. It has been asserted that the phenomena observed in these districts can only be accounted for by supposing that great fissures have opened in their midst, from which lavas have issued in enormous floods unaccompanied by the ordinary explosive phenomena of volcanoes. It must be remembered, however, that none of the districts in question have been subjected to careful and systematic examination with a view to the discovery of the vents from which these masses of lava have issued, with the exception of that which occurs in our own islands. In this case, in which superficial observers have spoken of the district as being covered with horizontal lava-sheets piled upon one another to the depth of 3,000 feet, careful study of the rock-masses has shown that the accumulations of basalt really consist of a great number of lava-currents which have issued at successive epochs covering enormous periods of time. During the intervals between the emission of these successive lava-currents the surfaces of the older ones have been decomposed, and formed soils upon which forests have grown up; they have been eroded by streams, the valleys so formed being filled with gravels; and lakes have been originated on their surfaces in which various accumulations have taken place. [Sidenote: TRULY-VOLCANIC ORIGIN OF LAVA PLATEAUX.] It has been demonstrated, moreover, that the basal-wrecks of no less than five volcanic mountains, each of which must have rivalled Etna in its proportions, existed within this area, and the connection of the lava-currents, which have deluged the surrounding tracts, with these great volcanoes has been clearly proved. It is probable that when more careful and systematic researches are carried on in the other districts, in which widely-spread sheets of basaltic rocks exist, similar volcanic vents will be discovered. It must also be remembered that if such a country as Iceland were subjected to long-continued denudation, the mountain peaks and cones of loose materials would be worn away, the whole island being thus reduced to a series of plateaux composed of lava-sheets, the connection of which with the crystalline materials filling the great volcanic vents, a superficial observer might altogether fail to recognise.