This volcano girdle is by no means a perfect one, for in addition to the principal festoons of the western border there are many secondary ones, and still other arcs are found well toward the center of the oceanic area. Another broad belt of volcanoes borders 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 neighboring deeps. [Illustration: FIG. 91.—Volcanic cones formed in 1783 above the Skaptár fissure in Iceland (after Helland).] [Illustration: FIG. 92.—Diagrams to illustrate the location of volcanic vents upon fissure lines, _a_, openings caused by lateral movement of fissure walls; _b_, openings formed at fissure intersections.] =Arrangement of volcanic vents along fissures and especially at their intersections.=—Within those districts in which volcanoes are widely separated from their neighbors, the law of their arrangement 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 exceptionally perfect line of small cones is found along the Skaptár 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 _gjás_, usually a few yards in width, but extending for many miles. The Eldgjá 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 conduits for the molten lava. This is well illustrated by the cratered monticules raised by the common ant over the cracks which separate the blocks of cement sidewalk, the hillocks being located where the most favorable channel was found for the elevation of the materials. [Illustration: