CHARACTERISTICS OF ROCKS. 63 



crystalline grains, and not water-worn particles or fragments of other 

 rocks. Examples : granite, gneiss, mica schist, basalt. 

 The crystalline rocks may have been crystallized — 



a. From fusion, like lava or basalt, when they are called igneous 

 rocks. Igneous rocks are often Called eruptive rocks, a term signifying 

 that they have been ejected from below, through fissures intersecting 

 other rocks. 



b. From solution ; as with the limestone called travertine. 



c. Through long-continued heat, usually without complete fusion. 

 By this last method, sedimentary beds, that is, those made originally 

 from mud, clay, etc., have been altered into granite, gneiss, or mica 

 schist, and compact limestone into statuary marble. Since, in such 

 cases, a bed originally sedimentary has been metamorphosed into a 

 crystalline one, rocks of this altered kind are called metamorphic 

 rocks. 



Distinctions of Rocks. — A few rocks consist of a single mineral 

 alone : as, for example, limestone, which may be either the species 

 calcite or dolomite ; quartzyte (along with much sandstone), which is 

 quartz; and felsyte, which is orthoclase. But even these simple kinds 

 are seldom free from other ingredients, and often contain visibly other 

 minerals. Nearly all kinds of rocks are combinations of two or more 

 minerals. They are not definite compounds, but indefinite mixtures, 

 and hardly less indefinite than the mud of a mud-flat. The limits be- 

 tween kinds of rocks are consequently ill-defined. Granite graduates 

 insensibly into gneiss, and gneiss as insensibly into mica schist and 

 quartzyte ; and so it is with many other kinds. This fact is a chief 

 source of the difficulty in studying and defining rocks, and especially 

 the crystalline kinds. 



In the study of any rock, the following are points to be deter- 

 mined : — 



1. Whether Fragmental or Crystalline in Texture. — This is easily 

 ascertained when the grains are coarse, those of fragmental rocks 

 having their edges rounded, and those of crystalline rocks having them 

 angular on a surface of fracture. But if fine-grained, it is best first to 

 examine the rock over a large area, and see whether it graduates into 

 coarser kinds that are obviously fragmental, or that contain a pebble 

 here and there, or that contain fossils ; or whether it is not so related 

 to other fragmental or sedimentary beds in position that the doubt is 

 thus removed ; or whether a coarser kind is not crystalline-granular 

 and hence a crystalline rock ; or whether its connection with eruptive 

 rocks in the region is not such as to settle the question. The examina- 

 tion of thin, transparent slices by the microscope will usually give 

 definite information when other methods fail. 



