866 PRINCIPLES OF STRATIGRAPHY 



to deep sea. The most stupendous modern examples of fissure 

 eruptions are those of eastern Iceland. In this island occur a 

 number of distinct and parallel clefts arranged in two dominant 

 series, one extending northeast and southwest, the other north and 

 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 

 i8 English miles and a depth varying from 400 to 600 feet." 

 ( Hobbs-i 5 :pp.) 



According to Thoroddsen, the lava wells out quietly from the 

 whole length of some of these fissures, overflowing on both sides 

 without the formation of cones. These fissures, therefore, consti- 

 tute connecting dikes, such as are known to occur under the older 

 lava flows of this type. At three of the wider portions of the great 

 Eld cleft of Iceland the lava has welled out quietly without the 

 formation of cones, flooding an area of 270 square miles. Upon 

 the southern narrower prolongation of the fissure, however, a 

 row of low slag cones appeared, and this is a feature characteristic 

 of other fissures in Iceland, as well as the great Skaptar fissure 

 reopened in 1783, emitting great volumes of lava. Subsequently the 

 eruptive processes became concentrated at the wider portions of 

 the fissure and a row of small cones was left over the line of the 

 fissure. Upon this fissure, too, stands the large volcano of Laki. 

 The great eruptions and the larger volcanoes are generally found 

 at the intersection of two fissures, as in the case of the great 

 eruption of Askja in 1875. and of the volcanoes of Java. On a 

 small scale, the formation of volcanoes along fissures is shown in 

 the frozen surface of the lava lake in the caldron of Kilauea, where 

 miniature volcanoes form whenever the crust which hardens in the 

 lava-lake becomes fissured. 



The connection of volcanic activities with fissuring of the earth's 

 surface is further shown in the great rift-valley of eastern Africa, 

 where extensive outpourings of lava have covered portions of the 

 valley floor, while volcanoes of great height and comparatively 

 recent origin have arisen within the valley, as in the case of the 

 Mfumbiro Mountains, already referred to, which block the rift- 

 valley north of Lake Kivu and which rise to great altitudes, the 

 crater rim of the still active volcano Kirungo-cha-Gongo rising to 

 11,350 feet above the sea-level, while Karisimbi reaches an altitude 

 approaching 14,000 feet. (Fig. 21, p. 125.) The valley floor 

 on which these volcanoes arose was considerably less than 4,000 

 feet above sea-level ; indeed, this same valley floor in the region of 

 Lake Tanganyika to the south actually descends below sea-level. 



