86 JOURNAL OF GEOLOGY— SUPPLEMENT 



on the possibility of this continual offsetting of the composition of 

 the liquid with its accompan3H[ng increased temperature range of 

 crystalHzation. 



On account of the cooHng effect of the dissolution of solid rock 

 the process should be limited from that cause/ Another important 

 consideration is the effect of the addition of the foreign material on 

 the fluidity of the magma. Exception must be taken to the state- 

 ment, sometimes made, that, as a result of assimilation the magma 

 becomes more fluid. In so far as the temperature is lowered the 

 magma becomes less fluid. In so far as the composition of the 

 magma is changed the fluidity may increase or decrease, depending 

 on the nature of the material added. In the case of the absorption 

 of a salic rock by a basic magma there would certainly be a very 

 important decrease of fluidity from both the above-mentioned 

 causes. The result would here be a marked restriction of differ- 

 entiation and not a tendency to promote it. 



The opinion that the fluidity of a magma is increased by the 

 dissolution of foreign material is probably the result of a failure to 

 distinguish between fluidity and fusibility. It is true that the 

 addition of one substance to another nearly always increases the 

 fusibility, i.e., there is a mutual lowering of melting-point, but 

 even this principle must be applied with caution and with due 

 regard for the exact nature of the dissolving liquid and the dissolved 

 material. Thus the addition of tin to lead lowers the tempera- 

 ture at which soKdification begins. The temperature at which 

 solidification is complete is, moreover, lower than the melting- 

 point of either of the metals separately. It is true also that if some 

 bismuth is added to this mixture of lead and tin the temperature of 

 beginning of solidification and the temperature of completion of 

 solidification will be still further lowered. But if to this mixture 

 of lead, tin, and bismuth a mixture of any two of these, say lead 

 and bismuth, is added, the temperature of beginning of soKdifica- 

 tion may be either raised or lowered, depending on proportions in 

 the mixtures, and the temperature of completion of solidification 

 will be affected not at all. The solution of a foreign rock in a magma 



' Normally one would expect the cooling effect to be great, the specific heats of 

 liquid silicates being so small as compared with latent heats. 



