142 EARTH FEATURES AND THEIR MEANING 



FIG. 150. The half-submerged volcano of Krakatoa 

 in the Sunda Straits before and after the eruption of 

 1883 (after Verbeek). 



sequent to the partial destruction of an earlier caldera, portions 

 of which were still existent in the islands Verlaten and Lang. 

 By the eruption of 1883 all the exposed parts and considerable 

 submerged portions of the two smaller cones were entirely de- 

 stroyed, and the larger one, known as Rakata, was divided just 

 outside the plug so as to leave a precipitous wall rising directly 



from the sea and 

 showing lava streams 

 in alternation with 

 somewhat thicker 

 tuff layers, the whole 

 knit together by nu- 

 merous lava dikes. 



In order to carry 

 our dissecting pro- 

 cess down to levels 

 below the base of the volcanic mountain, it is usually necessary to 

 inspect the results of erosion by running water. Here the plug or 

 chimney, instead of being surrounded by tuff, is inclosed by the 

 country rock of the region, which is commonly a sedimentary 

 formation. Such exposed lower sections of volcanic chimneys are 

 numerous along the northwestern shores of the British Isles. 

 Where aligned upon 

 a dislocation or note- 

 worthy fissure in the 

 rocks, the group of 

 plugs has been re- 

 ferred to as a sc*ar or 

 cicatrice (Fig. 151). 

 Associated with the 

 plugs of the cicatrice 

 are not infrequently 



dikes, or, it may be, sheets of lava extended between layers of 

 .sediment and known as sills. 



If we are able to continue the dissection process to still greater 

 depths, we encounter at last igneous rock having a texture known 

 as granitic and indicating that the process of consolidation was 

 not only exceedingly slow but also uninterrupted. This rock 

 is found in masses of larger dimensions, and though generally of 



FIG. 151. The cicatrice of the Banat (after Suess). 



