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GEOLOGY. 



THE FRAGMENTAL PRODUCTS OF SUDDEN COOLING. 



Pyroclastic rocks. — The extreme example of sudden cooling is pre- 

 sented when lavas are violently exploded into the air and solidify almost 

 instantly. The resulting glassy particles or filaments, if small, con- 

 stitute volcanic ash. The explosion appears to be due to steam and 

 other gases which are held in the deeper lava under great pressure, but 

 which, as they rise toward the surface of the lava where the pressure 

 is reheved, expand with explosive violence. It is probably also due in 

 part to progressive crystalHzation, which forces the gases out from the 



Fig. 336. — Volcanic bomb. About half natural size. (Photo, by Church.) 



part that crystaUizes and overcharges the rest. Sometimes the pro- 

 jected particles draw after themselves long filaments like the threads of 

 spun glass, and sometimes while in the air they divide and draw apart, 

 spinning a filament of viscid lava between them. A variety of this kind 

 at the volcano of Kilauea in Hawaii is known as ^'Pele's hair.'' These 

 light filaments drift with the wind and lodge on the lee side of the volcano^ 

 covering the surface ''like mown grass" (Dana). 



When the exploded fragments are coarser they fall about the volcanic 

 vent and form the tuffs (tufa) of which most steep volcanic cones are 



