SEGREGATION. 481 



mentary strata exhibit segregations of another kind. The 

 flints and the nodules of iron pyrites that are found in chalk, 

 as well as the silicious concretions which occasionally occur 

 in limestone, can be interpreted only as aggregations of 

 atoms of silex or sulphuret of iron, originally diffused al- 

 most uniformly through the deposit, but gradually collected 

 round certain centres, notwithstanding the solid or semi- 

 solid state of the surrounding matter. What is called bog 

 iron-ore supplies the conditions and the result in still more 

 obvious correlation. 



Among igneous changes we do not find so many exam- 

 ples of the process described. When distinguishing simple 

 and compound evolution, it was pointed out ( 102) that an 

 excessive quantity of contained molecular motion, prevents 

 permanence in those secondary re-distributions which make 

 evolution compound. Nevertheless, geological phenomena 

 of this order are not barren of illustrations. Where the 

 mixed matters composing the Earth's crust have been raised 

 to a very high temperature, segregation habitually takes 

 place as the temperature diminishes. Sundry of the sub- 

 stances that escape in a gaseous form from volcanoes, sub- 

 lime into crystals on coming against cool surfaces; and so- 

 lidifying as these substances do, at different temperatures, 

 they are deposited at different parts of the crevices through 

 which they are emitted together. The best illustration, 

 however, is furnished by the changes that occur during the 

 slow cooling of igneous rock. When, through one of the 

 fractures from time to time made in the solid shell which 

 forms the Earth's crust, a portion of the molten nucleus is 

 extruded ; and when this is cooled with comparative rapidity, 

 through free radiation and contact with cold masses ; it forms 

 a substance known as trap or basalt a substance that is uni- 

 form in texture, though made up of various ingredients. 

 But when, not escaping through the superficial strata, such 

 a portion of the molten nucleus is slowly cooled, it becomes 

 what we know as granite: the mingled particles of quartz, 



