MODE OF EXTRUSION 41 
her origin probably in a similar geological history of these parts 
of the Petehora plain in the sense of Suess (cf. p. 16); and a fur- 
ther consequence is a close concordance in time of eruption of 
both rocks. — Already Holmes fixed the basalts of King Charles 
and as being a link of the Arctic ones from a chemical standpoin t, 
and the writer once raised (l. c.) their common character with the 
 diabases of Eastern Spitsbergen in large content of enstatite-augite ; 
perhaps the age of their extrusion is somewhat later than the last 
mentioned, thus the low content of titanium dioxide do be constant 
from King Charles land to the Petchora plain and lower than in the 
_ older Spitsbergen rocks, lower than in the arctic basalts in general. 
No alkaline affinity seems to appear in this western part of the 
_ eastern plateau-half. 
3. MODE OF EXTRUSION OF THE EASTERN BASALTS. 
While the greater part of the Atlantic half of the Arctic plateau 
is buried beneath the sea, in consequence inaccessible to detailed 
investigations upon the mechanics of extrusion and not suitable to 
critical comparisons from locality to locality the geological fate 
of the parts being too variegated in their conditions, contrarily the 
Siberian half exposes a vaste land-suriace, which in her central part 
since proterozoic time has undergone no folding processes and the- 
efore may offer very uniform conditions with respect to magma- 
extrusions. | 
It is true that the part of the vaste plateau lying north off the 
Angara (Upper Tunguska) river is a difficult task to investigate and 
the general exploring ways follow the heading stream valleys, thus 
often giving a wrong idea of the geological structure of the tract 
between them. The beginning systematic geological mapping on 
the southern part of the area (Meister) affirms this statement, for 
