3i8 SCIENCE PROGRESS 



of which the magnetic sulphide of iron, pyrrhotite, often found 

 associated with norite, is the most important for the present 

 purpose. Mr. Harker, however, is disposed to explain the 

 segregation of corundum and the spinels from peridotites in 

 the same way. Moreover, Vogt's experiments were made on 

 anhydrous mixtures, and it does not follow that his results apply 

 to magmas rich in water and other volatile fluxes. It is true 

 that the elements of the alkali felspars appear to be miscible, even 

 at moderate temperatures, in all proportions with water, and the 

 same seems to be true of other magmas rich in alkalies, such as 

 pitchstone; there is nothing, however, to show that water is at 

 similar temperatures completely miscible with the majority of 

 silicates in the absence of a considerable amount of the alkalies. 



It is therefore possible that a magma rich in water and not 

 containing an excess of alkalies over the alumina might separate 

 on cooling into two layers, the first containing the greater part 

 of the water, the elements of quartz and the alkali felspars 

 with a small amount of the other constituents, the second con- 

 sisting mainly of the silicates of calcium, magnesium and iron. 1 



The separation of norite and granite in the saucer-shaped 

 igneous mass in the neighbourhood of Sudbury, Lake Superior, 

 is probably a case of gravitational differentiation anterior to 

 ■crystallisation. Here the total thickness is as much as a mile 

 and a quarter. Similar relations appear to exist between the 

 norite and red granite of the Bushveldt Plutonic Series of 

 the Transvaal, which is also enormously thick. In these cases 

 the great thickness would probably enable an effective separation 

 to take place even if the magma were continuous. If, however, 

 there were two discontinuous magmas, each would also suffer 

 a certain amount of continuous gravitational separation, so that 

 it might be difficult to say from an examination of the rocks 

 whether there had been a separation into two magmas or not. 



On the question of the classification of rocks Mr. Harker 

 agrees with Becker that it should be based on the eutectic ratios. 

 He would also pay special regard to rock structures which, like 

 " the pegmatoid, the graphic, and (in part) the porphyritic, have 



1 Arrhenius takes up a somewhat similar position, but Mr. Harker is inclined 

 to think that the experiments of Barus, who showed that glass was miscible with 

 water in all proportions at as low a temperature as 200 C, are inconsistent with 

 his views. Barus does not mention the composition of the glass he employed, but 

 it was doubtless ordinary glass containing a large percentage of alkali. 



