Invasion to Regional Metamorphism. '261 



a series of retreatal moraines and outwasli plains wMcli 

 mark glacial retreat, not glacial advance. 



"We must then seek to analyze in the batholithic border 

 phenomena what belongs to the advance and what to the 

 retreat of the magmatic state. 



Daly-^ has presented the evidence to show that the 

 gabbros have intrusive habits as laccoliths or sills, bodies 

 with floors, whereas the great granite bodies show no 

 evidence of floors. The magma on its first rise is ande- 

 sitic or basaltic in composition, and shows a greater power 

 of penetrating the crust, giving dominant extrusives; 

 the acidic phase of the magma differentiates later and 

 gives dominantly subjacent bodies. The oldest batho- 

 lithic phases are commonly a monzonite or diorite, the 

 gabbro reaching beyond only in occasional intrusive 

 bodies. The advance of the batholith is, then, especially 

 during the existence of the magma before it has become 

 highly differentiated. 



The differentiation and intrusion of later more acidic 

 bodies are phenomena of the waning stage. There are 

 evidences that granitic magmas pass through a stage of 

 high viscosity before becoming crystalline. In this 

 stage, when affected by crust movements, they assume 

 fluidal structures, granulate their feldspars, and shatter 

 the solid inclusions within them. These phenomena are 

 clearly features of decadence in magmatic invasion. If 

 the feldspathic infiltration of wall rocks is correctly 

 interpreted as a phenomenon of intercrystallization, this, 

 analogous to the outwash plains in front of a glacier, 

 may be both an antecedent and succedent feature, but the 

 difference is that in the advancing stage of magmatic 

 invasion, precipitation in an outer border must turn to 

 solution as the magma advances and hotter gases are 

 poured into the walls. The highly insinuating gases are 

 the advance agents and more normal magma follows in 

 the wider channels. A network of sheets rises into the 

 cover, it is dissevered into large fragments. In some 

 cases, especially those in Finland which Sederholm has 

 described, the sediments fade into the magma in place 

 and a new composite magma arises. This border 

 assimilation, however, as Daly has well shown,^^ must in 

 the nature of things be a limited process, and observation 

 here confirms the theory in showing that the batholithic 



-^ Igneous Eocks and tlieir Origin, 1914. 

 '' Op. cit., Chap. XI. 



