RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 101 



the great eruption of Askja in 1875 occurred at the intersection 

 of two lines of fissure. 



Outside these closely packed volcanic regions, similar though 

 less marked networks are indicated; as, for example, in and near 

 the Gulf of Guinea. If now, instead of reducing the scale of our 

 volcano maps, we increase it, the same law of distribution is no 

 less clearly brought out. The monticules or small volcanic cones 

 which form upon the flanks of larger volcanic mountains are like- 

 wise built up over fissures which on numerous occasions have 

 been observed to open and the cones to form upon them. 



Still further reducing now 

 the area of our studies and 

 considering for the moment the 

 " frozen " surface of the boil- 

 ing lava within the caldron of 

 Kilauea, this when observed at 

 night reveals in great perfec- 

 tion the sudden formation of 

 fissures in the crust with the 



. . FIG. 94. Map of the Puy Panou in the 



appearance Of miniature VOl- Auvergne of central France. The seat of 

 Canoes rising successively at eruption has migrated along the fissure 



more or less regular intervals " p wb j<* th a e earli f r cone had been 



built up (after Scrope). 



along them. 



It not infrequently happens that after a volcanic vent has 

 become established above some conduit in a fissure, the conduit 

 migrates along the fissure, thus establishing a new cone with more 

 or less complete destruction of the old one (Fig. 94). 



The so-called fissure eruptions. The grandest of all volcanic 

 eruptions have been those in which the entire length and breadth 

 of the fissures have been the passageway for the upwelling lava. 

 Such grander eruptions have been for the most part prehistoric, 

 and in later geologic history have occurred chiefly in India, in 

 Abyssinia, in northwestern Europe, and in the northwestern 

 United States. In western India the singularly horizontal pla- 

 teaus of basaltic lava, the Dekkan traps, cover some 200,000 

 square miles and are more than a mile in depth. The underlying 

 basement where it appears about the margins of the basalt is 

 in many places intersected by dikes or fissure fillings of the same 

 material. No cones or definite vents have been found. 



