122 EARTH FEATURES AND THEIR MEANING 



out in the lava before its rise in the chimney of the volcano, the 

 surrounding fluid lava may be blown to finely divided volcanic 

 dust which floats away upon the wind, thus leaving the crystals 

 intact to descend as a crystal rain about the crater. Such a 

 shower occurred in connection with the eruption of Etna in 1669, 

 and the black augite crystals may to-day be gathered by the 

 handful from the slopes of the Monti Rossi (Fig. 125, p. 125). 



The te,rm lapilli, or sometimes rapilli, is applied to the ejected 

 lava fragments when of the average size of a finger joint. This is 



the material which still 

 partially covers the un- 

 exhumed portions of the 

 city of Pompeii. Vol- 

 canic sand, ash, and dust 

 are terms applied in 

 order to increasingly 

 fine particles of the 

 ejected lava. The finest 

 material, the volcanic 

 dust, is often carried 

 for hundreds and some- 

 times even for thou- 

 sands of miles from the 

 crater in the high-level 

 currents of the atmos- 

 phere. Inasmuch as 

 this material is de- 

 posited far from the 

 crater and in layers 

 more or less horizontal, 



such material plays a small role in the formation of the cinder 

 cone. The coarser sands and ash, on the other hand, are the 

 materials from which the cinder cone is largely constructed. 



The manner of formation and the structure of cinder cones 

 may be illustrated by use of a simple laboratory apparatus (Fig. 

 119). Through an opening in a board, first white and then 

 colored sand is sent up in a light current of air or gas supplied 

 from suitable apparatus. The alternating layers of the sand 

 form in the attitudes shown; that is to say, dipping inward or 



FIG. 119. Artificial production of the structure of 

 a cinder cone with use of colored sands carried up 

 in alternation by a current of air (after G. Linck). 



