16 PRINCIPLES OF PALZONTOLOGY. 
at the mouths of our great rivers, and on a smaller scale wher- 
ever there is running water. Every stream, where it runs into 
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Fig. 4.—Sketch of Carboniferous strata at Kinghorn, in Fife, showing stratified beds 
(limestone and shales) surmounted by an unstratified mass of trap. (Original.) 
a lake or into the sea, carries with it a burden of mud, sand, 
and rounded pebbles, derived from the waste of the rocks 
which form its bed and banks. When these materials cease 
to be impelled by the force of the moving water, they sink to 
the bottom, the heaviest pebbles, of course, sinking first, the 
smaller pebbles and sand next, and the finest mud last. Ulti- 
mately, therefore, as might have been inferred upon theoretical 
grounds, and as is proved by practical experience, every lake 
becomes a receptacle for a series of stratified rocks produced 
by the streams flowing into it. These deposits may vary in 
different parts of the lake, according as one stream brought 
down one kind of material and another stream contributed 
another material ; but in all cases the materials will bear ample 
evidence that they were produced, sorted, and deposited by 
running water. The finer beds of clay or sand will all be 
arranged in thicker or thinner layers or laminz ; and if there 
are any beds of pebbles these will all be rounded or smooth, 
just like the water-worn pebbles of any brook-course. In all 
probability, also, we should find in some of the beds the re- 
