MATERIALS OF THE EARTH 



19 



Pyroclastic rocks have somewhat the structure of sedimentary 

 rocks. If the fragmental volcanic matter accumulates on the sur- 

 face of the land, it. often lacks distinct stratification; but if it falls 

 or is washed into water, it may be assorted and stratified. In this 

 case it is distinguished from clastic rock by its constitution. 



Minor structural and textural features of igneous rock. Most 

 igneous rocks are made up of interlocking crystals of different sorts. 

 These crystals may be so small that they are not readily distin- 

 guished by the eye, or they may be so large as to be easily seen, or 



Fig. 10 Porphyritic structure. P=phenocrysts of feldspar. The smaller 

 crystals are of feldspar, mica, and quartz. (Watts.) 



some may be large and some small. If they are large enough to 

 be distinct to the eye even without close scrutiny, the rock is coarsely 

 crystalline. It is proposed to call all such rocks phanerites. In the 

 case of phanerites, the interlocking of the crystals is evident (Fig. 9) . 

 If the crystals are so small as not to be readily seen except by the 

 help of a magnifying glass, the rock is an aphanite. In all igneous 

 rocks, the crystals are of somewhat unequal size; but in some, 

 there are certain crystals, usually of some one mineral, which are 

 much larger than the others so much larger as to be conspicuous. 

 The rock is then said to be porphyritic (Fig. 10). The smaller 

 crystals in which the larger ones are set may be so small as not to 

 be readily distinguished (aphanitic), or they may be separately 

 visible (phaneritic) . 



