STKATIFICATION. 81 



After these illustrations, the following definitions will be under- 

 stood. 



a. Stratification. — A succession of rock-layers, either of the same 

 or of different kinds. 



b. A layer. — ■ A single member or bed in a stratified rock. It may 

 be thick or thin, and loosely or strongly attached to the adjoining 

 layers. In the section, Fig. 60, the limestones 4 and 6 consist of 

 great numbers of layers ; and in all limestone regions many are piled 

 together to make the great mass of limestone. 



c. A stratum. — The collection of layers of one kind which form a 

 rock as it lies between beds of other kinds. In the section referred 

 to (Fig. 60), the limestones 4, 6, and the shale masses 3, 5, 7, are 

 each a stratum. A stratum may consist of many layers. 



d. A formation. — A series of strata comprising those that belong to 

 a single geological age, or to a single period or subdivision of an age, 

 and which, consequently, have a general similarity in their fossils or 

 organic remains. The Goal-formation includes many strata of sand- 

 stone, shales, limestones, and conglomerates. 



Geologists speak of the Silurian formation, Devonian formation, Carboniferous (or 

 Coal) formation, etc., making each cover a geological age. But they often apply the 

 term also to subordinate parts of these formations. Thus, under Silurian, we have the 

 Upper Silurian formation,nnd the Lower Silurian formation ; and under each of these 

 there are subordinate formations, as the Trenton formation, including the strata of the 

 Trenton epoch in the Lower Silurian ; the Niagara formation, for one of the lower sec- 

 tions of the Upper Silurian. These subdivisions embrace generally many strata, and 

 have striking peculiarities in their organic remains; and hence this use of the word 

 formation. 



e. A terrane. — This term is used for any single rock or continuous 

 series of rocks, of a region, whether the formation be stratified or not. 

 It is applied especially to metamorphic and igneous rocks, as a basaltic 

 terrane, etc. 



f. A seam is a thin layer intercalated among the layers of a rock, 

 and differing from them in composition. Thus, there are seams of 

 coal, of quartz, of iron-ore. Seams become beds, or are so called, when 

 they are of considerable thickness ; as, for example, coal-beds. 



These strata, which constitute so large an extent of the earth's crust, 

 have been formed mainly by the action of water. As the ocean now 

 makes accumulations of pebbles and sand, and muddy flats along its 

 borders, and muddy bottoms for scores of miles in width along various 

 sea-shores, so it formed, by the same means, many of the strata of sand 

 and clay which now constitute the earth's rocks ; and, in this work, the 

 sea often had the advantage, in early times, of sweeping widely over 

 the just-emerging continent. Again, as the rivers bring down sand 

 and mud. and spread them in vast alluvial flats, making deltas about 

 6 



