370 GEOLOGY 



hurling portions of lava to great heights and shattering them into 

 fragments, special forms of which are called bombs, cinders, ash, 

 etc., all of which constitute pyrodastic material. Much of the ex- 

 plosive violence of volcanoes has been attributed to the contact 

 of the hot rising lava with ground-water, but the function of ground- 

 water in the explosions has probably been exaggerated. 



There are two phases of extrusion, and at their extremes they 

 are strongly contrasted. The one is explosive ejection, often 

 attended with great violence; the other a quiet out-welling of the 

 lava. More or less closely related to these two phases of extrusion 

 are two classes of conduits, the one, restricted openings, often pipes, 

 ducts, or limited fissures, from which the amount of lava extruded 

 is relatively small, and hence it congeals near the orifice, forming 

 cones; the other, great fissures out of which the lava pours in 

 great volume and from which it spreads over wide tracts, often in 

 broad thin sheets. There is no fundamental difference between 

 great fissure eruptions and the eruptions of restricted vents, and 

 the two types blend. The extent of the spreading of lava into thin 

 sheets is due more to the mass and the fluidity of the lava than to 

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

 periods appear to have issued mainly from extended fissures. 



a. Fissure Eruptions 



The chief known fissure eruptions of recent times are the vast 

 basaltic floods of Iceland; but at certain times in the past there 

 have been prodigious outpourings of lava, flow following flow 

 until formations thousands of feet thick and covering thousands 

 of square miles, were built up. One of these occurred in Tertiary 

 times in Idaho, Oregon, and Washington (Fig. 290), where about 

 200,000 square miles were covered with sheets of lava, aggregating 

 in places some 2,000 feet in thickness. Still earlier, in the Cretaceous 

 period, there were enormous flows on the Deccan, covering a like 

 area to the depth of 4,000 to 6,000 feet. Still earlier, in the Ke- 

 weenawan period, an even more remarkable succession of lava- 

 flows in the Lake Superior region developed a series of igneous 

 rocks of almost incredible thickness. In these cases there is little 

 evidence of explosive or other violent action. There are but few 



