JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VII MAY 19, 1917 No. 10 



GEOLOGY. — On the terms aphrolith and dermoliih. T. A. 

 Jaggar, Jr., Hawaiian Volcano Observatory. (Communi- 

 cated by Arthur L. Day.) 



Dutton 1 in describing Hawaiian volcanoes made use of the 

 terms aa and pahoehoe, and these words of Hawaiian origin have 

 been somewhat adopted by American geologists. Some Eng- 

 lish geologists object to them strongly and continental usage is 

 various in the matter. Aa lava of the Hawaiians is charac- 

 terized by a peculiar mechanism of solidification of olivine 

 basalt, whereby a magma surcharged with gas so froths or foams 

 in its surface layer that on solidification this layer shrivels and 

 separates into discrete scoriaceous units, in some places light 

 and pumiceous, but elsewhere heavy and slaggy. The extreme 

 pumiceous type is the limu, or " thread-lace scoria" of Dana. 

 The lava flow beneath solidifies as a coarsely vesicular body 

 grading downward into crystalline rock which has no known 

 petrographical peculiarity different from any other type of 

 flow. The boundary surface at the top of the continuous sheet 

 and below the discontinuous layer of scoriae is revealed at many 

 places on the shoreline of Hawaii, where the ocean has swept 

 the loose material away. In these places the surface revealed 

 is rugose and in detail much like the scoriaceous fragments, 

 but it sometimes exhibits coarse ropy forms and rough festoons 



1 Dutton, C. E. Hawaiian Volcanoes. 4th Ann. Rep. U. S. Geol. Surv., 

 1882-83. 



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