LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 87 



is in a general way analogous to this latter case. It is an addition 

 to the magma of materials which it already contains. Either a 

 raising or a lowering of the temperature of beginning of crystalliza- 

 tion may result as in the case of the alloys mentioned. On account 

 of the prevalence of solid solution among the constituents of rocks 

 even the temperature of completion of crystallization may also be 

 either raised or lowered. 



Experiments along this line have been made by Petrasch. In 

 one of these, two parts of limburgite were mixed with one part of 

 granite and a supposed lowering of freezing temperature found.^ 

 But the melts in this case cooled partly to glass and the so-called 

 Erstarrungspunkt is merely the observer's opinion as to the point at 

 which this glass became sufficiently viscous to be called a rigid body. 

 This temperature has nothing to do with the temperature of 

 change from liquid to solid, i.e., crystals, and this change is the 

 significant factor in the present connection, for only in the case of a 

 magma which cooled sufficiently slowly to crystallize can the possi- 

 bility of its having absorbed surrounding rocks be even seriously 

 entertained. Probably the actual temperature range of crystalliza- 

 tion of the mixture mentioned above would be intermediate between 

 the corresponding ranges of the individual rocks, a condition which 

 would certainly be true for the mixing of any two rocks in which 

 feldspar mix-crystals were important constituents. This may be 

 taken as a very general, though possibly not universal, feature of 

 the mixing of polycomponent rocks. 



Neither increased fluidity nor increased fusibihty can be safely 

 postulated as an ever-present aid to the continuance of assimila- 

 tion and to the differentiation of the syntectic. 



It may therefore be repeated that the result of assimilation 

 should normally be an obviously hybrid rock. Recent studies in 

 the Pyrenees, where assimilation on a large scale has been claimed, 

 have led to the conclusion that assimilation is Kmited in amount 

 and that the rocks produced are unmistakably hybrid.^ 



^ Neues Jahrb., Beil. Band XVII (1903), 508. 



^ See O. H. Erdmannsdorffer, " Petrograpliioche Untersuchungen an einigen 

 Granit-Schieferkontakten der Pyrenaen," Neues Jahrb., Beil, Band XXXVII (1914), 

 740. 



