go 



University of California. 



LVOL. 2. 



tite as at any other point, while the feldspar in the latter is appar- 

 ently due more to resorption than to a lack of differentiation. The 

 extreme variations in the gabbro are due to secondary differentia- 

 tion, while the dikes of intermediate character in the peridotite rep- 

 resent either a differentiation of the latter or a partial resorption of 

 the feldspathic facies at a greater depth. Where differentiation has 

 taken place on a small scale, as at Point Morrito, resulting in 

 stringy masses and nodules of the different constituents only a few 

 feet in diameter, it would seem that the phenomenon could not be 

 due to differences of temperature and pressure, but, under favor- 

 able circumstances of great liquidity and slow cooling, to a tendency 

 to segregation of similar molecules through chemical affinity. 



The theory which Rosenbusch* has advanced to account for the 

 differentiation of homogeneous magmas seems to be applicable 

 more than any other to the phenomena observed here. He con- 

 ceives that the original magma within the earth was homogeneous, 

 and that it separated into partial magmas of different composition 

 according to the laws of chemical affinity. The two extremes of 

 this process he terms the foyaite (NaK)AISi.,, and the peridotite 

 (MgFe)Si magmas, with an intermediate one (CaAl 2 Si 4 ). As the 

 last increases in amount, the " metal kern," representing the peri- 

 dotite magma, becomes more easily soluble in that of the foyaite 

 magma. 



There is certainly shown here a tendency to the separation of 

 certain molecules independent of differences in temperature. One 

 of these is rich in magnesia and iron, the other in calcium and 

 alumina. 



Geological Laboratory \ 



University of California, May i, i8r>6. 



*Min. u. petr. Mitth., Vol. XI, pp. 144-178. 



