868 



PRINCIPLES OF STRATIGRAPHY 



vast number of dikes, which represent the filling of the fissures 

 through which the lava reached the surface. 



A Cretacic example of such outpouring of basic lava, rivaling 

 in extent that of the northwestern United States, is seen in the great 

 bed of Deccan trap which forms the surface of the Deccan Plateau 

 in India. Here the depth of the lava is from 4,000 to 6,000 feet. 

 Where the basement rocks on which this trap sheet rest are exposed 

 by erosion along the margin of the plateau dikes of basalt are seen 

 penetrating them, representing in part the fissures through which 



Fig. 230. End of the lava flow of 1881 near Hilo, Hawaiian Islands. The 

 lava surface is a typical pahoehoe surface. (After Button. ) 



the lava reached the surface. No cones or definite vents have been 

 found. 



What appears to be a pre-Palaeozoic example of such eruptions 

 is seen in the great Keweenawan lava sheets which represent a pro- 

 longed succession of outpourings in the Lake Superior region, 

 aggregating an enormous amount variously estimated as reaching 

 the great thickness of 15,000 or 25,000 feet. Here, too, there is 

 little evidence of explosive or other concentrated volcanic activity. 



The lava dotnc. Where eruptions are concentrated about a 

 single opening a mountain of lava will be built up which rises in 

 proportion to the frequency of the eruption and the volume of lava 

 poured out. Where fragmental material is absent, as in the Ha- 

 waiian volcanoes, the slope is a very gentle one, though the actual 



