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The formation and detachment of a nebulous ring, illustrates the samegeneral principle. To conclude, as Laplace did, that the equatorial portionof a rotating nebulous spheroid will, during concentration, acquire a centrifugalforce sufficient to prevent it from following the rest of the contractingmass, is to conclude that such portions will remain behind as are in commonsubject to a certain differential force. The line of division between thering and the spheroid, must be a line inside of which the aggregative forceis greater than the force resisting aggregation; and outside of which theforce resisting aggregation is greater than the aggregative force. Hencethe alleged process conforms to the law that among like units, exposed tounlike forces, the similarly conditioned separate from the dissimilarly conditioned. §165. Those geologic changes usually classed as aqueous, displayunder numerous forms the segregation of unlike units by a uniform incidentforce. On seashores the waves are ever sorting-out and separating the mixedmaterials against which they break. From each mass of fallen cliff, the tidecarries away all those particles which are so small as to remain long suspendedin the water; and, at some distance from shore, deposits them in the shapeof fine sediment. Large particles, sinking with comparative rapidity, areaccumulated into beds of sand near low water-mark. The small pebbles collecttogether at the bottom of the incline up which the breakers rush; and onthe top lie the larger stones and boulders. Still more specific segregationsmay occasionally be observed. Flat pebbles, produced by the breaking downof laminated rock, are sometimes separately collected in one part of a shinglebank. On this shore the deposit is wholly of mud; on that it is wholly ofsand. Here we find a sheltered cove filled with small pebbles almost of onesize; and there, in a curved bay one end of which is more exposed than theother we see a progressive increase in the massiveness of the stones as wewalk from the less exposed to the more exposed end. Trace the history ofeach geologic deposit, and we are quickly led down to the fact that mixedfragments of matter, differing in their sizes or weights, are, when exposedto the momentum and friction of water, joined with the attraction of theEarth, selected from one another, and united into groups of comparativelylike fragments. And we see that, other things equal, the separation is definitein proportion as the differences of the units are marked. After they havebeen formed, sedimentary strata exhibit segregations of another kind. Theflints and the nodules of iron pyrites that are found in chalk, as well asthe silicious concretions which sometimes occur in limestone, are interpretedas aggregations of molecules of silex or sulphuret of iron, originally diffusedthrough the deposit, but gradually collected round centres, notwithstandingthe solid or semi-solid state of the surrounding matter. Bog iron-ore suppliesthe conditions and the result in still more obvious correlation.

Among igneous changes we do not find so many examples of the process described.

Nevertheless, geological phenomena of this order are not barren of illustrations.

Where the mixed matters composing the Earth's crust have been raised to avery high temperature, segregation commonly takes place as the temperaturefalls. Sundry of the substances that escape in a gaseous form from volcanoes,sublime into crystals on coming against cool surfaces; and solidifying, asthese substances do, at different temperatures, they are deposited at differentparts of the crevices through which they are emitted together. The best illustration,however, is furnished by the changes that occur during the slow cooling ofigneous rock. When, through one of the fractures from time to time made inthe Earth's crust, a portion of the molten nucleus is extruded, and whenthis is cooled with comparative rapidity, there results trap or basalt --a substance that is uniform in texture, though made up of various ingredients.

But when, not escaping through the superficial strata, such a portion ofthe molten nucleus is slowly cooled, granite is the result: the mingled particlesof quartz, feldspar, and mica, being kept for a long time in a fluid andsemi-fluid state -- a state of comparative mobility-undergo those changesof position which the forces impressed on them by their fellow units necessitate.

The differential forces arising from mutual polarity, segregate the quartz,feldspar, and mica, into crystals. How completely this is dependent on thelong-continued agitation of the mixed particles, and consequent long-continuedmovableness by small differential forces, is proved by the fact that in agranite dyke the crystals in the centre, where the fluidity or semi-fluiditycontinued for a longer time, are much larger than those at the sides, wherecontact with the neighbouring rock caused more rapid cooling and solidification. §166. The actions going on throughout an organism are so involved,that we cannot expect to identify the forces by which particular segregationsare effected. Among the few instances admitting. of interpretation, the bestare those in which mechanical pressures and tensions are the agencies atwork.