305 
neighbouring centres from which lavas were poured out over 
the roof of the batholite, the thickness of the lava cover would 
be materially augmented, and if stoping was still in progress 
the batholitic magma would invade this new part of the cover’. 
If processes going on within the batholitic chamber resulted in 
the formation of a magma different from that existing at the 
time of breaking through, a further complication obscuring the 
record of the breaking through might arise. The possibility of 
such a partial replacement of one batholitic rock by another 
must be allowed if the overhead stoping hypothesis be accepted; 
as Daty and others have pointed out there is a probability that 
such replacements have actually occurred in some cases, and 
this will also appear from the discussion given below of the 
internal structure of the llimausak batholite. 
We thus conclude that if a batholitic magma on one or 
more occasions during its intrusion has penetrated its cover, 
the incident, as a rule, will be obliterated by the continuation of 
the stoping process. The presence of a sedimentary cover, as 
at Marysville, shows that outbursts from the batholitic magma 
can only have occurred through fissures in the cover, but ba- 
tholites of this kind are relatively rare. More commonly the 
cover entirely consists of lavas belonging to the same cycle of 
igneous activity as does the batholite itself (e. g. Boulder ba- 
tholite of Montana”), and several of the large Plutonic masses 
of the Christiania district, and of the west of Scotland etc. In 
such cases the observed phenomena are not, as a rule, inconsi- 
stent with the assumption that the batholitic magma has broken 
through, and has even taken part in the production of the very 
lava sheets into which the consolidated batholite is intrusive. 
1 Of the porphyry covering the Ilimausak batholite it is not possible to 
decide, whether it is derived from Ilimausak or from Igaliko or from 
some other unknown centre. 
? J. Barrett, Geology of the Marysville Mining District, 1907, р. 167. 
XXX VIII. 20 
