480 SEGREGATION. 



than the aggregative force. Hence the alleged process con- 

 forms to the law that among like units, exposed to unlike 

 forces, the similarly conditioned part from the dissimilarly 

 conditioned. 



165. Those geologic changes usually classed as aque- 

 ous, display under numerous forms the segregation of unlike 

 units by a uniform incident force. On sea-shores, the waves 

 are ever sorting-out and separating the mixed materials 

 against which they break. From each mass of fallen cliff, 

 the rising and ebbing tide carries away all those particles 

 which are so small as to remain long suspended in the 

 water; and, at some distance from shore, deposits them in 

 the shape of fine sediment. Large particles, sinking with 

 comparative rapidity, are accumulated into beds of sand 

 near low-water mark. The coarse grit and small pebbles 

 collect together on the incline up which the breakers rush. 

 And on the top lie the larger stones and boulders. Still 

 more specific segregations may occasionally be observed. 

 Flat pebbles, produced by the breaking down of laminated 

 rock, are sometimes separately collected in one part of a 

 shingle bank. On this shore the deposit is wholly of mud ; 

 on that it is wholly of sand. Here we find a sheltered cove 

 filled with small pebbles almost of one size; and there, in a 

 curved bay one end of which is more exposed than the other, 

 we see a progressive increase in the massiveness of the stones 

 as we walk from the less exposed to the more exposed end. 

 Trace the history of each geologic deposit, and we are 

 quickly led down to the fact, that mixed fragments of 

 matter, differing in their sizes or weights, are, when ex- 

 posed to the momentum and friction of water, joined 

 with the attraction of the Earth, selected from each other, 

 and united into groups of comparatively like fragments. 

 And we see that, other things equal, the separation 

 is definite in proportion as the differences of the units are 

 marked. After they have been formed, sedi- 



