PYROGENIC ROCKS 277 



C-3. Vesicular, or puiniccous and scoriaceous. 

 When rendered light and celkilar by the 

 abundance of steam holes due to expansion 

 of the steam during solidification. The 

 commoner, rougher type constitutes the 

 scoriaceous texture. These vesicles may 

 characterize the entire mass or be limited 

 to layers or streaks. In the basic rocks the 

 scattered vesicles may become filled by min- 

 eral matter either during or subsequent to 

 the volcanic period, and so produce an 

 amygdaloidal structure. 



C-4. Sphcrulitic, microsphcriilitic, variolitic, etc. 

 Containing globes or spherules made up of 

 extremely delicate fibers of feldspar, ar- 

 ranged radially and embedded in a mass 

 consisting chiefly of aggregates of tridy- 

 mite or quartz. In size these spherulites 

 may range to several inches. Microsphcni- 

 lites are very minute spherulitic bodies, 

 often occurring in great numbers in bands 

 or affecting the entire ground mass. The 

 large spherulites of older acid lavas are 

 often partly or totally replaced by flint or 

 quartz. In basic rocks a similar structure 

 is known as variolitic. Lithophyscc are 

 spherulitic structures varying up to a foot 

 or more in diameter, and consisting of con- 

 centric shells separated by interspaces. 

 (For illustrations, see Iddings-15; pis. xli 

 and xliii.) 



Cryptocrystalline and Acrystalline rocks may also exhibit a 

 banded texture, i. e., flow structure on a small scale (see beyond). 

 From the combination of any member of the composition group 

 Avith members of the texture group we get the common types of 

 pyrogenic rocks, as follows* : 



* For a more detailed discussion of the pj^rogenic rocks the student is referred 

 to the Hand Book of Rocks, by J. F. Kemp, and to the work of Cross, Iddings, 

 Pirsson and Washington, on Quantitative Classification of Igneous Rocks, cited 

 in the Bibliography. See also Harker, A Natural History of the Igneous 

 Rocks, and for a brief treatise, his Petrology for Students, Cambridge. 



