310 SCIENCE PROGRESS 



other considerations which can only be briefly alluded to here, 

 but which will be found clearly explained in detail in Mr. 

 Harker's book. 



Under certain circumstances, which have been investigated 

 by Principal Miers and other workers, a mineral does not begin 

 to separate out when the temperature of saturation is reached, 

 and the magma may become supersaturated to a considerable 

 extent before crystallisation begins. It can be shown that this 

 may result in the crystallisation alone of first one and then the 

 other mineral before the two crystallise out together. In some 

 cases, too, the crystals which were first formed may be partly 

 " resorbed " while the second mineral is in process of crystal- 

 lisation. 1 



Another modification is introduced in cases where two 

 substances are capable of forming mixed crystals or " solid 

 solutions," for combinations of this character separate out in 

 the place of the simple minerals. If the two substances are 

 imperfect isomorphs, each can only hold in solid solution a 

 certain percentage of the other; thus orthoclase can contain 

 not more than 28 per cent, of albite, and albite not more than 

 12 per cent, of orthoclase. The succession of events in this 

 case presents considerable analogies to that of two substances 

 that are not isomorphs, but the composition of the crystals that 

 separate out is dependent on, though not identical with, the 

 composition of the magma at the moment they are formed. A 

 eutectic magma is finally formed which solidifies with the 

 simultaneous crystallisation of orthoclase containing a maximum 

 of albite, and albite containing a maximum of orthoclase. It 

 is believed that the well-known perthitic intergrowth of these 

 minerals, in which orthoclase contains streaks and patches of 

 albite, or in some cases vice versa, is produced in this manner. 2 



Where two substances are perfect isomorphs, as in the case 

 of albite and anorthite, they can form mixed crystals in every 

 proportion, and the final product is no longer a eutectic mixture 

 of two distinct kinds of crystals. 



1 The symbolical representations of events given on p. 214 are not quite 

 complete. They should read (A) ; (B) (-A); (B) ; (AB) and (B) ; (A) (-B); 

 (A) ; (AB). 



2 In some instances, however, it is believed to be a secondary structure due 

 to the decrease, with the temperature, of the capacity of one mineral to hold 

 the other in solid solution. 



