92 LITHOLOGICAL GEOLOGY. 



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

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

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

 or organic remains. The Coal formation includes many strata of sand- 

 stone, shales, limestones, and conglomerates. 



While this is always the general' idea connected with the term formation, the 

 use of it is not uniform. Geologists speak of the Silurian formation, Devonian 

 formation, Carboniferous (or Coal) formation, etc., making each cover a geo- 

 logical age. But they often apply the term also to subordinate parts of these 

 formations. Thus, under Silurian we have the Upper Sihirian formation and 

 the Lower Silurian formation ; and under each of these there are subordinate 

 formations, as the Trenton formation, including several strata of the Trenton 

 period in the Lower Silurian ; the Niagara formation for the lower part 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 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. 



99. 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, 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 their mouths thousands 

 of square miles in area, so in ancient time beds of sand and clay 

 were accumulated by these very means and afterwards consolidated 

 into rocks. Again, as shells and corals, by growing in the ocean 

 where shallow, under the action of the waves, produce the accumu- 

 lating and rising coral-reef some hundreds of miles long in the pre- 

 sent age, so in former ages shells and corals grew and multiplied and 

 made coral-reefs and shell-rocks, and these old reefs are the lime- 

 stone strata of the world. The agency of water and life in these great 

 results is particularly considered under Dynamical Geology. 



2. Structure of Layers. 

 The structure of layers is due either to the original deposition 

 of the material, or to subsequent changes. 



100. (1.) Kinds of structure and markings originating in the 



