78 



occur only as the margins of basalt-dykes or in basalt-tuffs. In the Sandwich 

 Islands, however, they are developed on a large scale in the crater of Kilauea. 



APPENDIX TO GROUP F. 



The most important change which affects the glassy rocks is that known 

 as devitrification. By this process the older acid glasses have been frequently 

 changed tofelsite. 



Palagonite is a substance formed by the alteration and hydration of 

 basalt-glass. 



GROUP G. 



FRAGMENTAL VOLCANIC ROCKS. 



Fragmental volcanic rocks are termed ashes, tuffs and agglomerates. 

 No special nomenclature is required for rocks of this group. The character 

 of the ash, tuff or agglomerate in any given case may be indicated by the 

 term applied to the corresponding lava. We thus speak of liparite- or 

 rhyolite-tuffs, andesite-tuffs, basalt-tuffs, &c. 



APPENDIX TO GROUP G. 



The fragmental rocks, like their massive representatives, are, of course, 

 subject to change by surface and deep-seated agencies, and also by regional- 

 metamorphism. 



