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VOLCANOES 



A mass of lava, when it cools and solidifies, necessarily contracts, 

 and since the cohesion of the mass is insufficient to allow it to 

 contract as a whole, it must crack into blocks, separated by 

 fine crevices, which are called joints. The mutual relations of 

 the jointing planes, and the consequent shape of the blocks, are 

 determined largely by the grain of the lava and its degree of 



Fig. 14. — Stream gorge, island of Hawaii; displaying modern columnar lava. 

 (Photograph by Libbey.) 



homogeneousness. In fine-grained (and some coarse-grained) 

 homogeneous lavas the jointing is apt to be very regular, and to 

 give rise to prismatic or columnar blocks, which are usually hex- 

 agonal. This shape is due to the fact that the formation of hex- 

 agons requires less expenditure of work than other figures, and is 

 produced by the intersection of systems of three cracks, radiating 

 from equidistant points at angles of 120 . The long axes of the 

 prisms are at right angles to the cooling surface. Starch and 

 fire-clay, which shrink on drying, joint in the same way. The 



