303 
On the other hand the existence of the cover is directly 
endangered by the stoping process. How does the process 
come to an end before the earth surface is reached, and why 
does the magma ‘never break through’? Barret, discussing this 
difficulty, comes to the conclusion that ‘although depending on 
many factors, the key to the permanence of the cover appears 
to lie in the difference in temperature between the cover and 
the magma, and in the viscous nature of the magma when cooled 
nearly to the point of solidification’!. Harrer” speaks of the fact 
that batholites ‘never break through’ as an obvious difficulty 
when overhead stoping is considered as a more than subsidiary 
factor in the mechanics of intrusion. 
In some respects, however, the correctness of these ob- 
jections does not appear to be sufficiently borne out by obser- 
vation. There is of course no question that overhead stoping 
does in some cases come to an end before the magma reaches 
the earth surface. Thus the Marysville batholite, as described 
by Barrett in the important monograph cited above, evidently 
had a permanent cover of sedimentary rocks, and the same may 
be true of several other batholites, but this fact cannot be 
considered as a grave objection to the hypothesis. Overhead 
stoping must, no doubt, be a very slow process. It requires 
a continual supply of heat from below, and when the magma 
nears the earth surface the consumption of heat will go on at 
a rapidly increasing rate, and the process, thus, will be more 
and more retarded. The escape of water vapour and other vo- 
latile substances through fissures will probably be the chief 
method by which heat is lost, and as soon as the loss of heat 
exceeds the supply from below, the magma will become more 
and more viscous, until the process gradually ceases. Since 
batholitic magmas are of acid or intermediate composition, loss 
1 Un. St. Geol. Surv., Professional Paper No. 57, р. 174 (1907). д 
? Natural History of Igneous Rocks (1909), р. 86. /‹ 
