572 PRINCIPLES OF STRATIGRAPHY 



Volcanic Dust Deposits. 



As has been shown in an earHer chapter (11), the fine dust re- 

 sulting from explosive eruptions of volcanoes is carried far and 

 wide over the surface of the earth and becomes a part of many 

 ■ widespread contemporaneous deposits. Of the i8 cubic kilometers 

 (4.3 cubic miles) of volcanic dust and cinders thrown into the air 

 on the explosion of Krakatoa in 1883, one-third fell at a distance 

 of more than 15 kilometers or 9.4 miles from the seat of disturb- 

 ance. At a distance of 1,000 miles from the volcano ashes still fell 

 inches deep. Shaler has estimated that not less than 300 cubic 

 miles of fine dust have been discharged by the Javanese and 

 Malayan volcanoes since 1770, and more than an equal quantity of 

 such dust has been thrown into the air by other volcanoes of the 

 earth during the same period. The settling of this material all over 

 the surface of the earth forms an important addition to the eolian 

 rocks of the crust. 



In character this volcanic tlust consists of minute angular grains 

 of vitreous or glassy material, often showing by their curved form 

 that they are parts of the walls of glass bubbles. This material has 

 been recognized in deposits of various ages among the stratified 

 series of the \arth's crust. Extensive deposits of anemopyrolutytes 

 and pyrarenytes have been found in the Tertiary of South America, 

 especially in Patagonia, where the remains of mammals are found 

 in it. Sinclair (47) has found that the Bridger (Eocenic) beds of 

 Wyoming are largely composed of such material, though cross- 

 bedded fluviatile deposits of sanidine and other sands are likewise 

 characteristic. The fine pyrolutyte of the Florissant basin is an ex- 

 ample of deposition of volcanic dust under playa lake or river flood 

 plain conditions with the entombment of many remains of terrestrial 

 organisms. Pyrolutytes and pyrarenytes are common in many 

 regions of the earth, but the conditions of their deposition are not 

 fully understood in most cases.* 



Special Indicators of Eolation. 



Rocks of terrestrial origin are not infrequently characterized by 

 the presence in their mass of wind-carved pebbles or glyptoliths.f 

 These are commonly dreikanter, and they indicate that for a time 

 at least the region containing them was above sea-level and subject 



* See further under pyroclastic rocks, Chapter XII. f Woodworth, 53a. 



