406 GEOLOGY. 



rounded projectiles are known as volcanic bombs (Figs. 336 and 337). 

 Balls of lava that have originated in rolling movements of the seeth- 

 ing mass, or in other ways, are also styled bombs. Usage is not 

 altogether harmonious or consistent in the application of the term. 



The larger masses that are projected into the air are more or less 

 vesicular from the expansion of included gases, as already noted, and 

 so the fragmental products of volcanic action gi'ade into the vesicular. 

 The type of this class is pumice, in which the gas cavities make up by 

 far the larger part of the volume of the whole mass, and the whole is 

 reduced to the condition of a solidified froth or foam. So thin are the 

 dividing films of glassy material in some cases that the whole is pure 

 white, though the same material in sohd mass would be dark. This 

 solidified glassy froth is often fighter than water and floats freely on the 

 sea until it becomes '^ water-logged " and sinks. Dredgings of the deep 

 sea show that much pumice has accumulated there, and being far from 

 the land has escaped burial by the sediments borne in by the rivers. 



All of these fragmental rocks produced by volcanic action are known 

 as pyroclastic (fire-fragmented) rocks, a general term of much con- 

 venience in distinguishing them from lava-flows, on the one hand, and 

 from the fragmental rocks produced by air and water (ordinary elastics), 

 on the other. 



THE GLASSY RCCKS. 



The solid glasses. — The quick cooling of lava-flows into sohd glasses 

 is chiefly dependent on their exposure at the surface. Hence it is often 

 the case that the exterior of a lava-flow is glassy in greater or lesser 

 degree, while the interior is more or less crystalhne. Quick coofing is 

 sometimes also due to the intrusion of the lava in thin sheets into fissures 

 in cold rocks. When massive bodies of lavas penetrate solid rocks, the 

 lava does not usually cool so fast as to prevent some degree of crystal- 

 lization, and the crystallization may even become complete; but if the 

 intruded lava sheet be very thin, the lava is liable to be cooled to a 

 nearly perfect glass. The glassy condition is, therefore, subject to 

 indefinite gradations. As a rule, the acid lavas are stiffer at the same 

 temperature than the basic ones, and crystalfize more slowly, so that 

 acid glasses are more common than basic ones. The basic rocks 

 usually crystallize pretty thoroughly, except on the immediate sur- 

 face of the flows. 



