LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 55 



single, principal act of intrusion of basaltic magma with subse- 

 quent differentiation practically in place and controlled in a general 

 way by the size and rate of cooling of the individual bodies formed. 

 The sequence of intrusion observed is the result of a sequence of 

 consolidation^ and of movements, resulting in a certain amount of 

 injection of one type into another, which may be wholly of a minor 

 nature and quite subordinate as compared with the main act of 

 intrusion of basaltic magma. In some localities more than one 

 principal intrusion of basaltic magma took place, and therefore two 

 or more interlocking gabbro-granite sequences may exist, with re- 

 sultant irregularities in the order of succession. 



The wholly random spatial arrangement of granite, grano- 

 diorite, monzonite, etc., often observed in the larger bodies plainly 

 cannot be the result of differentiation alone, whatever kind of 

 differentiation one may have a leaning toward.' The arrangement 

 is the result of movements following differentiation and should not, 

 by itself, be made the basis of either affirmation or denial of gravi- 

 tative differentiation. Nevertheless, the differentiation is of the 

 type which in other cases is clearly due to a separation of liquid 

 from crystals (fractionation),^ either by the sinking of crystals or 

 by the squeezing out of residual liquid, a fact which is sufficient to 

 establish a strong presumption in favor of these processes even when 

 they are not demonstrable. 



THE ALKALINE ROCKS 

 NEPHELITE SYENITES AND RELATED TYPES 



Biotite granite is, however, not necessarily the end or pole in 

 the process of differentiation which has been outlined. The 

 process may end and, very often, does end at this stage just as it 

 sometimes ends at the granodiorite or diorite stage if the cooling is 

 at the proper rate. Indeed, if magmas were anhydrous, we may 

 conclude from the information furnished by the investigated sys- 

 tems that a granite would be the necessary end beyond which 

 differentiation could not proceed; not a biotite granite, however, 



' See Ussing, Geology of the Country around Julianehaab, Greenland, p. 312, 

 2 See A. Knopf, U.S. Geol. Survey, Bull. 52'/, p. 35, on the origin of aplites associ- 

 ated with quartz monzonite of the Boulder bathoKth. 



