THE ORIGIN AND DESCENT OF ROCKS. 



411 



are carried. By an increase of the crystals in number and size, the rock 

 passes by gradations into porphyry or phanerite. 



Amygdaloids. — In lava-flows the included steam often collects in 

 bubbles near the surface as the lava cools and forms a vesicular portion 

 with a scoriaceous texture (Fig. 341). In its upper part, the vapor 

 bubbles may be numerous, while below they become more and more 

 scattered until they disappear. Similar bubbles are also often found 



Fig. 343. — Porphyritic texture. Natural size. (Photo, by Church.) 



near the bottom of a sheet of lava. This is perhaps due to the rolling 

 under of the frontal surface of the lava-stream as it flows. Later, these 

 cavities often become filled mth minerals deposited from solution and 

 the rock then becomes an amygdaloid, but this filling is a secondary 

 action. 



THE PORPHYRITIC ROCKS. 



When the conditions are such that after a part of the magma has 

 formed distinct crystals floating in the remaining liquid lava, there 



