RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 99 



secondary ones, and still other arcs are found well toward the 

 center of the oceanic area. Another broad belt of volcanoes bor- 

 ders the Mediterranean Sea, and is extended westward into the 

 Atlantic Ocean. Narrower belts are found in both the northern 

 and southern portions of the Atlantic Ocean, on the margins of 

 the Caribbean Sea, etc. The fact of greatest significance in the 

 distribution seems to be that bands of active volcanoes are to be 

 found wherever mountain ranges are paralleled by deeps on the 

 neighboring ocean floor (Fig. 90). As has been already pointed 

 out in the chapter upon earthquakes, it is just such places as these 

 which are the seat of earthquakes; these are zones of the earth's 

 crust which are undergoing the most rapid changes of level at the 

 present time. Thus the rise of the land in mountains is proceeding 

 simultaneously with the sinking of the sea floor to form the neigh- 

 boring deeps. 



Arrangement of volcanic vents along fissures and especially at 

 their intersections. Within those districts in which volcanoes 



FIG. 91. Volcanic cones formed in 1783 above the Skaptar fissure in Iceland 



(after Helland). 



are widely separated from their neighbors, the law of their arrange- 

 ment is difficult to decipher, but the view that volcanic vents are 

 aligned over fissures is now supported by so much evidence that 

 illustrations may be supplied from many regions. An excep- 

 tionally perfect line of small cones is found along the Skaptar 

 cleft in Iceland, upon which stands the large volcano of Laki. 

 This fissure reopened in 1783, and great volumes of lava were 

 exuded. Over the cleft there was left a long line of volcanic 

 cones (Fig. 91). There are in Iceland two dominating series of 

 parallel fissures of the same character which take their directions 

 respectively northeast-southwest and north-south. Many such 

 fissures are traceable at the surface as deep and nearly straight 

 clefts or gjds, usually a few yards in width, but extending for many 

 miles. The Eldgja has a length of more than 18 English miles 

 and a depth varying from 400 to 600 feet. On some of these 

 fissures no lava has risen to the surface, whereas others have at 

 numerous points exuded molten rock. Sometimes one end only 

 of a fissure, the more widely gaping portion, has supplied the 



