TEXTURES 



I 9 I 



bodies of water and are there sorted and stratified and, it may be, 

 mingled with more or less sand and mud and other truly sedi- 

 mentary material. Rocks formed in this manner partake of the 

 nature of both the igneous and sedimentary classes, and may be 

 referred to either group, or regarded as a series intermediate 

 between the other two and in a measure connecting them. These 

 rocks will here be treated as a special subdivision, under the name 

 of pyroclastic rocks. 



In our studies of the products of modern volcanoes, we saw that 

 the same molten mass will give rise to rocks of very different 

 appearance in its different parts, according to the circumstances 

 of rapidity of cooling, pressure, etc. We may now express this in 

 somewhat more general form and say that the texture of an igne- 

 ous rock is determined by the several factors which affect the 

 molten mass during consolidation. Of such factors may be men- 

 tioned the chemical composition, temperature, rate of cooling, 

 degree of pressure, and the quantity present of dissolved vapours 

 and gases, which are called mineralizers . In one and the same 

 continuous mass of rock we also find great differences of minera- 

 logical composition, a process of segregation taking place in the 

 molten magma. When this occurs, it is the general rule that the 

 mass becomes more basic toward the periphery. 



Chemical composition determines the fusibility of a rock at a 

 given temperature. The least fusible rocks are, on the one hand, 

 those which contain large quantities of silica, 60 to 75% (acid 

 rocks), and, on the other, those which contain less than 40% of 

 silica (ultrabasic rocks). The most fusible rocks are those with 

 an intermediate percentage of silica (basic rocks), and among these 

 the fusibility increases, as the percentage of silica diminishes, until 

 the lower limit is reached. The effect of chemical composition 

 upon texture is seen in the rapidity with which the less fusible 

 rocks chill and stiffen, and therefore the greater frequency with 

 which they form glasses. 



Chemical composition is, however, important in this connection 

 chiefly through its effect upon the rate of solidification. We have 

 already learned (p. 12) that solidification very generally takes 



