Invasion to Regional Metamorpliism. 5 



through geological time that process which operated so 

 widely in the Archeozoic and destroyed by batholithic 

 invasion, perhaps everywhere, the older foundations of 

 the earth. 



Harker^^ makes the valuable distinction between the 

 character of igneous intrusion in plateau and in mountain 

 X)rovinces. In the first, the intrusions have not been 

 connected with great compressive movements; in the 

 second, they are so related, and this determines a dif- 

 ferent habit in the forms of the intrusive bodies. Harker, 

 hoAvever, does not go into the problem of the granite 

 gneisses and their contacts as exhibited in provinces 

 marked by regional metamorphism. In fact, he favors 

 Broegger's view that the granite batholiths are nothing 

 more than large and irregular laccoliths. 



In the classification of batholiths according to the 

 environing conditions which existed at the time of their 

 solidification, we may draw two distinctions. First, they 

 may have approached to within a few thousand feet of 

 the surface of the earth, into a zone of fracture and rapid 

 cooling, or they may have ceased their upw^ard progress 

 at a depth of miles, w^hile still in the zone of flow and 

 Avhere their heat and their emanations would persist and 

 affect the roof above them for at least a geological period. 

 Those coming to rest at a depth of a few miles may be- 

 come exposed in later ages by profound erosion. Those 

 which lie deeper, if there be such, can never be recognized, 

 except indirectly by an interpretation of their effects. 



Secondly, we may divide batholiths, follomng the sug- 

 gestion of Harker, into those whose rise has not been 

 accompanied by regional deformation, and, on the other 

 hand, those during whose rise great regional deformation 

 has prevailed. To illustrate these categories: The 

 Boulder batholith of Montana is a body whose exposed 

 portion covers about 1100 square miles, and whose cover 

 over broad areas consisted merely of a somewhat earlier 

 series of extrusive andesites a few thousand feet in thick- 

 ness.- Some folding of the older sedimentary rocks seems 

 to have gone 'on before the invasion of the batholith, but 

 the latter shows no effects of compression. The dis- 

 turbances at the time of its origin and later seem to have 

 been essentially movements of vertical adjustment. 



In New England, on the other hand, the granites are 



^A. Harker, Natural History of Igneous Eocks, Chap. Ill, 1909. 



