248 



MATERIALS AND THEIR ARRANGEMENT 



is known as flow structure (Fig. 234). On cooling, some lavas 

 develop columnar structure (Fig. 235), the columns being roughly 

 perpendicular to the surface of cooling. 



The explanation of the columns is probably somewhat as fol- 

 lows: The surface of the lava contracts about equally in all direc- 

 tions on cooling. The contraction may be thought of as centering 

 about equidistant points. About a given point, the least number 

 of cracks which will relieve the tension in all directions is three 

 (a, Fig. 236, A). If these radiate symmetrically from a point, the 

 angle between any two is 120, the angle of the hexagonal prism. 

 Similar radiating cracks from other centers (6, c, etc.) complete the 

 columns (Fig. 236, B). A five-sided column would arise from the 

 failure of cracks to develop about one of the points. 



Igneous rocks are affected by cracks or joints, which run through 



them in various directions, 

 but this is not a feature 

 peculiar to igneous rocks. 

 Pyroclastic rocks have 

 somewhat the structure of 

 sedimentary rocks. If the 

 fragmental volcanic matter 

 accumulates on the surface 

 of the land, it may lack 



distinct stratification; but if 

 . ,. ,. . , , . 



it falls ^ or IS washed into 



water, it may be assorted 

 and stratified. In this case 

 it is distinguished from other clastic rock by its constitution. 



Textural features. Most igneous rocks are made up of interlock- 

 ing crystals of different sorts. These crystals may be so small that 

 they are not distinguished readily by the eye, or they may be so large 

 as to be seen easily, or some maybe large and some small. If they are 

 large enough to be distinct to the eye even without close scrutiny, the 

 rock is coarsely crystalline. All such rocks may be called phanerites. 

 In phanerites, the interlocking of the crystals is evident (Fig. 237). 

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

 rock is 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 so much larger than the others as to be 

 conspicuous. The rock is then porphyritic (Fig. 238). The smaller 



Fig. 236. Diagrams to illustrate the for- 

 mation of columns of basalt: A, the nrst 

 stage in the development of a hexagonal 

 column. B, the completion of a hexagonal 



column - 



