298 
into quartz bearing contact modifications at the junction with 
the sandstone. Since assimilation is dependent upon the temp- 
erature, it must be assumed that it has operated on a larger 
scale in the deeper portions of the batholite which are not ex- 
posed to view. It must also be borne in mind that the effects 
of the assimilation may become concealed or obscured by sub- 
sequent differentiation of the magma. 
The above considerations will show that, although the as- 
similation hypothesis does not suffice to account for the me- 
chanism of intrusion of the Ilimausak batholite, yet it is prob- 
able that the intrusion has taken place with the co-operation of 
assimilation processes. 
(V.) Finally we have to consider the hypothesis of batho- 
litic invasion by subsidence of roof blocks. This hypothesis 
postulates a long continued breaking and sinking into the 
magma of fragments of the roof rock, and a corresponding rise 
of the magma to occupy the place of the sunken fragments. 
The hypothesis agrees with the assimilation hypothesis in as- 
suming a passive and slow invasion from below, and the pro- 
cess does not necessarily imply any movements of the earth 
surface. It differs from the assimilation hypothesis in the 
assumption that the older rock is removed as fragments which 
pass down to unknown depths, leaving the chemical composition 
of the upper part of the magma uninfluenced. Brôccer has 
drawn attention to the fact that roof blocks must in some 
cases have sunk through the magma!; but the theory that a 
process of this kind may be the main cause of batholite inva- 
sions was first set forth in a more elaborate form by Dary in 
19032, who termed the process ‘overhead stoping’. According 
1 W. С. Broacrr, Die Eruptivgesteine des Kristianiagebietes II, р. 145, 
(1895). 
> В. A. Dany, Geology of the Ascutney Mountain. Un. St. Geol. Surv. 
Bull. No. 209, p. 93 (1903). — Idem. The Mechanics of Igneous Intru- 
sions. Amer. Journ. of Science 4th series XV (1903), XVI (1904), and 
XXVI (1908). 
