THE ROCKS OF THE EARTH'S SURFACE SHELL 33 



needle-like in form, as is very often the case, they arrange them- 

 selves " end on " during the rock flow, so that when consolida- 

 tion has occurred, the rock has a kind of puckered lamination which 

 is the characteristic of the fluxion or flow texture. This texture 

 has sometimes been confused with the lamination of the sedi- 

 mentary rocks, so that wrong conclusions have been reached 



FIG. 17. Characteristic textures of igneous rocks, a, granitic texture characteristic 

 of the deep-seated intrusive rocks ; 6, porphyritic texture characteristic of the ex- 

 trusive and of the near-surface intrusive rocks ; c, glassy texture of an extrusive rock. 



regarding origin. At other times the same needle-like crystals 

 within the lava have grouped themselves radially to form rounded 

 nodules called spherulites. Such nodules give to the rock a 

 spherulitic texture, which is nowhere better displayed than in the 

 beautiful glassy lavas of Obsidian Cliff in the Yellowstone Na- 

 tional Park. 



Those intrusive rocks which consolidate deep below the earth's 

 surface, part with their heat but slowly, and so the process of 

 crystallization is continued without interruption. Starting from 

 many centers, the crystals continue to grow until they mutually 

 intersect in an interlocking complex known as the granitic tex- 

 ture (Fig. 17 a). 



Classification of rocks. In tabular form rocks may thus be 

 classified as follows : 



Igneous. Massive and 



with sharply angular 

 ing 



Granitic or porphyritic texture. 



Glassv or porphyritic texture; 

 often also with vesicular, sconaceous, pumi- 

 ceous, fluxion, or spherulitic textures. 



