Park. — Border-segregation in some Igneous Magmas. 487 



and outward, and in the outer portion downward and inward. 

 This cycle of flow would apparently be as likely to form 

 central as border aggregations, or aggregations only in the 

 outer lower limits of the magma. 



Convection currents even in a very liquid magma would be 

 very slow, except there was a constant accession of heat from 

 below in amount greatly above that normal to the depth. Of 

 such an accession in the case of a magmatic mass forming 

 either a dyke or laccolite there is no evidence whatever. 



I am inclined to ascribe border-segregation mainly to 

 differences of osmotic pressure in the magma, with perhaps 

 convection currents as a contributing cause. Osmotic pressure 

 is a form of energy of great intensity. When precipitation 

 takes place from a homogeneous solution of dissolved salts it 

 instantaneously establishes a condition of equal concentration 

 throughout the whole mass. 



It may be urged, and not without reason, that an igneous 

 magma is not a homogeneous body in the same sense that a 

 solution of auro-potassic cyanide is said to be homogeneous. 

 For a magma is composed of watery vapour, various gases, 

 and a solution of solid constituents. It forms, however, a 

 two-phase system the constituents of which are homogeneous 

 in themselves, and consequently in a state of equilibrium at 

 constant temperature and pressure.* 



According to Van t'Hoff's law, the osmotic pressure of a 

 substance in solution is the same pressure which that sub- 

 stance would exert were it in gaseous form at the same 

 temperature, and occupying the same volume. 



That is, v = Rt 



where p — osmotic pressure in pounds per square inch ; 



R = the gas constant = 1,206 lb. per square inch ; 

 t = absolute temperature, centigrade ; 

 V = volume of the solvent containing one molecular 

 weight of the solute. 



And V = 10 °™ 



r 



where M = the sum of the atomic weights of the atoms 

 in a molecule of the dissolved substance ; 

 r = the strength per cent, of the solution. 



Therefore p = ...„ ' ■ ■ 



or p 



100 M 

 12r (t° + 273) 

 M 



* Professor W. Ostwald, " Elements and Compounds," Faraday Lecture 

 delivered before the Fellows of the Chemical Society in the Theatre of 

 the Royal Institution, London, 19th April, 1904. 



