THE ROCKS OF THE EARTH'S SURFACE SHELL 31 



rocks having been produced from magmas, which are essentially 

 homogeneous, are usually without definite directional structures 

 due to an arrangement of their constituents, and are said to have 

 a massive structure. Sedimentary rocks, upon the other hand, 

 have been formed by an assorting process, the larger and heavier 

 fragments having been laid down when there was high velocity of 

 either wind or water current, and the smaller and lighter frag- 

 ments during intermediate periods. They are therefore more or 

 less banded, and are said to have a bedded or laminated structure 

 (Fig. 16). 



Again, igneous rocks, being due to a process of crystallization, 

 are composed of mineral individuals which are bounded either 

 by crystal planes or by irregular surfaces along which neighboring 

 crystals have interfered with each other; but in either case the 

 grains possess sharply angular boundaries. Quite different has 

 been the result of the attrition between grains in the transpor- 

 tation and deposition of sediments, for it is characteristic of the 

 sedimentary rocks that their constituent grains are well rounded. 

 Eolian sediments have usually more perfectly rounded grains than 

 subaqueous deposits. 



Glacial deposits, if laid down directly by the ice, are unstrati- 

 fied, relatively coarse, and contain pebbles which are both faceted 

 and striated. Such deposits are described as till or tillite. If 

 glacier-derived material is taken up by the streams of thaw 

 water and is by them redeposited, the sediments are assorted 

 or stratified, and they are described as fluvio-glacial deposits. 



The metamorphic rocks. Both the coarser structures and 

 the finer textures of the metamorphic rocks are intermediate 

 between those of the igneous and the sedimentary classes. A 

 metamorphosed sedimentary rock, in proportion to its alteration, 

 loses the perfect lamination and the rounded grain which were 

 its distinguishing characters ; while an igneous rock takes on in 

 the process an imperfect banding, and the sharp angles of its 

 constituent grains become rounded off by a sort of peripheral 

 crushing or granulation. Metamorphic rocks are therefore 

 characterized by an imperfectly banded structure described as 

 schistosity or gneiss banding, and the constituent grains may be 

 either angular or rounded. If the metamorphism has not been 

 too intense or too long continued, it is generally possible to deter- 



