105 
only form a relatively thin cover from 200 to 300 meters in thick- 
ness over abyssal rocks, which have consolidated later than 
the porphyries. We can hardly assume that abyssal rocks have 
crystallised at a depth of but 200 to 300 meters under the 
earth-surface, and from this point of view we must conclude, 
therefore, that the volcanic rocks now existing must originally 
have been covered by igneous, or sedimentary, sheets of very 
considerable thickness. A still more direct proof is found in 
the numerous, large and regular dykes which traverse the whole 
of the still existing part of the volcanic series, showing that 
volcanic rocks must have composed at least an important part 
of the masses which have been entirely removed by erosion. 
We are thus led to the conclusion, that the red sandstone in 
the Ilimausak region has originally been covered by a series of 
voleanic sheets with a total thickness which probably amounted 
to several kilometers, but of the whole of this formation only 
a very small part has escaped destruction by erosion, the part, 
namely, which has been altered by contact-metamorphism round 
the abyssal rocks and has thus gained a special power of 
resistance. 
