THE EXTRUSIVE PROCESSES. 593 



There are two phases of extrusion often quite strongly contrasted. 

 The one is explosive ejection, often attended \\'ith great \iolence; the 

 other, a quiet out- welling of the lava, with little more than ebullition. 

 More or less closely related to these differences are two classes 

 of conduits, (a) the one. great fissures, out of which the lava pours in 

 great volume and spreads forth over wide tracts, often in broad thin 

 sheets; (b) the other, restricted openings, often pipes, ducts, or hmited 

 fissures, from which the extrusion is usually much less abundant, and 

 hence it more largely congeals near the orifice, forming cones. Flows 

 from the former constitute massive eruptions; those from the latter, 

 the more famihar volcanic eruptions. There is no radical difference 

 between them, and the two classes blend. The extent of the spreading 

 of lava into thin sheets is due more to the mass and the fluicUty than to 

 the form of the outlet. The stupendous outflows of certain geologic 

 periods appear to have issued mainly from extended fissures, doubt- 

 less because these better accommodated the outbiu'sting floods. 



a. Fissure eruptions. — The cliief knowm fissure eruptions of recent 

 times are the vast basaltic floods of Iceland. Most of the eruptions 

 of historic times are of the volcanic type; but at certain times in the past 

 there were prodigious outpomings, flow foUo^ing flow until layers 

 thousands of feet tliick covering thousands of square miles were built 

 up. One of these occurred in Tertiary- times in Idaho, Oregon, and 

 Washington, where some 200.000 square miles were covered with sheets 

 of lava, aggregating in places 2000 feet or more in thickness. Earlier 

 than this, in Cretaceous times, there were enormous flows on the Deccan 

 plateau of India, covering a like area to a depth of 4000 to 6000 feet. 

 Still earher than this, in Keweenawan times, an even more prolonged 

 succession of lava-flows covered nearly all the area of the Lake Superior 

 basin, and extended beyond it, and built up a series of almost incredible 

 thickness, the estimates reaching 15.000 to 25.000 feet. In these cases 

 there is little e\'idence of explosive or other violent action. There 

 are few beds of ash. cinders, and similar pyroclastic material. The 

 inference is. therefore, that the lavas weUed out rather quietly and 

 spread themselves rather fluently over the surrounding coimtry. For 

 the most part these wide-spreading flows are composed of basic mate- 

 rial, which is more easily fusible and more highly fluent at a given tem- 

 perature than the acidic lavas. The latter are more disposed to form 

 thick embossments near the point of extrusion. 



