96 



IGNEOUS AGENCIES. 



Many other volcanoes are known which have similar circular ram- 

 parts made up of layers of volcanic ejections. One of the most remark- 



Fig. 82.— Mount Vesuvius in 1756 (after Scrope). 



able of these is Barren Island, in the Bay of Bengal (Fig. 83). The 

 difference between this and Vesuvius is, that the circle is more com- 

 plete.* 



Fig. 83. 



-Section across Barren Island (after Mallett): s I, sea-level, 

 show the supposed former condition. 



The dotted line is added to 



Comparison between a Volcanic Cone and an Exogenous Tree.— It 



is evident, then, that a cone of eruption grows by layers successively 

 applied on the outside. Both in structure and growth it may, there- 

 fore, be compared to an exogenous tree : 1. As the sap ascends 

 through the center of the shoot and descends on the outside, forming 

 layers of wood, one outside of the other, increasing 

 every year the height and the diameter of the tree ; so 

 in a volcano lava ascends through the center and pours 

 over the outside, forming also successive layers, in- 

 creasing both the diameter and the height. 2. As a 

 cross-section of a tree shows concentric rings around 

 (Fig. 84) a central pith, and is traversed by pith-rays ; 

 so a cross-section of a volcano would show a central crater, with con- 

 centric layers, traversed by radiating dikes. 3. As on the pith-rays, 

 where they emerge upon the surface, arise buds, which grow in a man- 

 ner similar to the trunk ; so on the radiating dikes are formed monti- 



Fig. 84. 



Medlicot and Blandford, Manual of Geology of India, p. 736. 



