302 
in the localization of the intrusion. Nevertheless, at Ilimau- 
sak the latest subsidence of the overlying earth crust took 
place after the intrusion of the batholite, but this, as men- 
tioned below, was a purely local subsidence, being probably 
caused by the diminution of volume of the magma during cry- 
stallization. 
The original thickness of the cover. — When the igneous 
activity came to an end, the Ilimausak batholite had a cover con- 
sisting of tuffs, lava sheets, and sills, which probably had a 
considerable thickness. How great was this thickness cannot 
be acertained, for the quantity of rock material which has been 
removed from the surface during the long continued period of 
erosion which followed the volcanic activity is unknown. The 
remnants still remaining on the lofty peaks of Ilimausak 
doubtless represent only a small part of the cover which 
once existed. In the reconstructed section, Fig. 26 (p. 290), 
the cover has been given a thickness of a little more than one 
kilometer, but this is quite an arbitrary estimate. 
The cover which existed at the time of cessation of the 
igneous activity is not, however, necessarily the original cover. 
It has been pointed out by several authors that the hypothesis 
here adopted to account for the mechanism of intrusion in- 
volves the question of the maintenance of the cover. This 
problem may be regarded from two different points of view. 
From the statical point of view it is difficult to imagine 
how a batholitic magma of very large horizontal dimensions 
could support a cover consisting of a material heavier than it- 
self. In the present instance, however, the horizontal dimen- 
sions are not so large that the cover would be in danger of 
foundering if stoping ceased at a distance of one kilometer or 
more from the surface, and as the question has been elabo- 
rately discussed by Daty' from a general point of view, it may 
here be left out of consideration. 
1 Amer. Journ. of Science (4) XXVI, p. 30 (1908). 
