VOLCANIC PRODUCTS 



41 



differences are due to the varying degrees of accumulated pressure, 

 the resistance to be overcome, the character of the lava, the 

 manner in which the steam is generated, and similar factors. 



Volcanic Products. — These form the most important part of the 

 subject from the geological point of view, because they contribute 

 largely to the permanent materials of the earth's crust. We meet 





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1- V--- - - 



Fig. 9. — Another view of the crater-floor and walls of Kilauea. (Photograph 



by Libbey.) 



with such materials of all geological ages, sometimes developed on 

 a vast scale. The study of volcanic products is the key which 

 enables us to comprehend the great group of rocks which are 

 called igneous, though, as we shall see later, by no means all of 

 these were poured out on the surface of the ground. 



Volcanic products are of three kinds: (1) lava, or molten 

 rock; (2) frag7iiental matei'ial, including blocks, lapilli, bombs, 

 the so-called volcanic ashes, cinders, and the like ; (3) gases and 

 vapours. 



