24 PHYSICAL GEOLOGY 



ample, may be large and easily distinguished (Fig. 7), or 

 minute and unrecognizable. They may be of the same size, 

 or of very unequal sizes (Fig. 8). These are matters of tex- 

 ture, rock texture having to do with the size, shape, and ar- 

 rangement of the particles of a rock. 



FIG. 8. Porphyritic texture. About % natural size. 

 (Photograph by Baker.) 



Chemical classes of igneous rocks. Those which con- 

 tain a large proportion of silica (more than 65 per cent) are 

 called acidic rocks, because silica is an acid-forming oxide 

 (uniting with water to form silicic acid). Of the other lead- 

 ing oxides (p. 19) none commonly form acids. Similarly, 

 those igneous rocks which contain much less silica (less than 

 55 per cent) and a larger proportion of the bases (lime, soda, 

 magnesia, potash, etc.) are called basic rocks. Most acidic 

 rocks are light-colored if crystalline, while basic rocks are 

 commonly dark-colored. An intermediate or neutral group is 

 sometimes recognized, including rocks that contain 55 to 65 

 per cent of silica. 



