A, 



VOLCANOES AND IGNEOUS INTRUSIONS 331 



77. Compact or Fine-grained Igneous Rocks 



In this group are included rocks in which the grains are so fine that 

 the individual crystals cannot be distinguished by the naked eye. 

 They are intermediate between the granitoid rocks, composed of 

 clearly distinguishable crystals, and the glasses. No definite line 

 can be drawn between the two groups; in some dikes, for example, 

 a glass shades imperceptibly into a microcrystalline rock, and then 

 into a coarsely crystalline or granitoid rock. 



This group is divided into two classes on the basis of color : (1) the 

 light felsites and (2) the dark basalts. . 



(1) Felsites vary greatly in color, but are not dark gray, dark green, 

 or black. To the naked eye the rock has a flinty aspect, but with a 

 lens it is often seen that it consists of mineral grains, too small for 

 determination. When large crystals (phenocrysts) occur embedded 

 in the fine-grained " ground mass," the rock is called a felsite 

 porphyry. Porphyries contain feldspar phenocrysts (Greek, phain- 

 esthai, to appear, and krystallos, crystal). If quartz is also present, 

 they are known as quartz porphyries y and if hornblende is con- 

 spicuous, they are called hornblende porphyries. Felsites occur in 

 dikes and sheets. 



(2) Basalts form a very large and important group of igneous rocks. 

 They are all heavy black, gray, brown, or greenish rocks of fine tex- 

 ture, and have a wide distribution, covering many thousands of square 

 miles of the earth's surface. The name trap is also used to include 

 basalts and any dark-colored, heavy igneous rocks whose mineral 

 constituents have not been determined. 



When the air cavities of vesicular basalts or of other igneous rocks 

 are filled with minerals, the rocks are called amygdaloidal. This is 

 one mode of occurrence of copper in some of the mines of northern 

 Michigan (p. 396). 



777. Glassy Rocks 



Rocks which are composed partly or wholly of glass are included 

 in this group. They were formed as stated (p. 329), when molten 

 rock solidified rapidly. They are therefore lavas which were either 

 poured out on the surface, or in crevices where they were subjected to 

 rapid cooling. One sometimes finds the sides of dikes glassy, while 

 the interior is crystalline. The texture of glassy rocks is sometimes 

 vesicular (Fig. 294) and sometimes pumiceous. 



