In various parts of the earth and at different geological horizons are large areas covered by very extensive, generally horizontal, series of sheets of basaltic lavas, the series of overlying flows often attaining thicknesses of thousands of meters. In some cases they are accompanied by flows of rhyolite. These basalts have poured out in an evidently very fluid condition, as they occupy preexisting valleys and cover the lower topographic features much like floods of water, the separate flows being very long—many of them measured by miles.

It is generally assumed by volcanologists that these extensive, horizontal, very fluid flows have issued quietly from fissures—an idea first suggested by Sir Archibald Geikie.2 Volcanic cones, formed of lavas, ashes, or both, are present in places, but these are, inconspicuous, being low because of the fluidity of the lavas, and they always form a very minor feature of the complex.

Such lavas . . .

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