12 THE JOURNAL OF GEOLOGY. 



III. If they are volcaiiic : 



1. They may appear to be striped, banded, or pseudo- 

 " stratified" conformably to adjoining sedimentary 

 deposits ; 



2. They will probably be accompanied by fragmental 

 (pyroclastic) material, which may or may not itself 

 be really stratified. Such material will vary greatly 

 in coarseness, containing bombs, agglomerates, brec- 

 cias, tuffs, sands and ashes. The characteristics of 

 these are : 



i) indiscriminate mixture of all sizes and shapes of 

 fragments ; 



2) material of same kind as the igneous rocks ; 



3) cement, either finer fragmental material (tuff- 

 breccia) or lava (flow-breccia); 



4) very angular shape of smallest fragments (micro- 

 scopic glass sherds). 



5) if ancient volcanoes were on the shore-line, such 

 material may have been immediately worked over 

 by water and interbedded with more or less 

 normal aqueous sediments. 



IV. Most important of all, however, is the identification of 

 those characteristic structures known to originate only 

 in glassy, half-glassy or very fine grained porphyritic 

 rocks, solidifying at the surface, or in very narrow dykes 

 where solidification has been rapid. These will be found 

 to be very persistent and can usually be identified under 

 the microscope in spite of devitrification, alteration, or 

 even a considerable degree of dynamometamorphism. 

 The most common of these structures are : 



1. a vesicular, scoriaceous, pumiceous or amygdaloidal 

 structure ; 



2. a sharply defined, small porphyritic structure with a 

 glassy, half-glassy or felsitic (cryptocrystalline) base; 



3. a spherulitic structure, due to either large or small 

 lithopysae, hollow spherulites, or compact spherulites. 



