36 ARCTIC BASALT PLATEAU 
feldspars begin to separate following the decreasing lime content. 
If the magma is steam-overladen, the bulk of alkalies from the 
resorbed lenads can be redissolved during the uninterrupted epi- 
magmatic and postmagmatic flux-rising stage of the rock-develop- 
ment immediately pursuing the magmatic one; and then deposited 
in the steam cavities or elsewhere as amygdale minerals. In this 
manner the rock loses her primary or secondary alkalic character | 
and becomes an ordinary basalt, if the cooling was not going too 
fast and steam or gas was not wanting. If in exchange the cooling 
was going faster and the mineral separation was interrupted by 
quenching in some part y of the hypothetic curve figured as ap- 
pendix to x, the lenadic mineral composition may be fixed and 
consists of minerals in some regard instable.! 
The two cases are realised by a rock body of moderate dimen- 
sions intruded on a gentle depth in a sedimentary series of at least 
some permeability to gases; and by a rock of surface-extrusion or 
in general of quick cooling. The basaltoid nephelinite shows all 
signs of surface extrusion, while the Bennet-land trachydolerite gives 
by her coarser grain and the local circumstances some signs of an 
intrusive sheet. These two examples do not imply an exceptional 
primary alkaline character of the magma, contrarily a slight one; 
but they give an explanation of the sporadic alkaline character of 
the Arctic basalts and this explanation can be extended to the | 
young volcanics of Spitsbergen. 
B. Basalt from the nearest east. 
The geological features of the volcanics of the Petchora plain are 
related in a former chapter (p. 16). The petrological character of 
this basalt is to be described. 
1 The raising steam content to the end of crystallisation probably gives such a 
direction to the crystallisation way, and the true end of this curve is signified by 
particular redissolution of the feldspars, the latter being broken down and giving 
place to the zeolite formation as early as in epimagmatic stage. 
