32 Pentti Eskola. (LXTII 
Si0, being less than 1:6 causes the appearance of norma- - ; 
tive nephelite, as is often the case in rocks free from quartz 
but with abundant biotite. 
The hornblende is of the common green variety 
having rather dark colours. The biotite also has dark 
colours (mark the high ratio FeO : MgO in the rock). 
The mica is idiomorphic towards the hornblende and both Sd 
are idiomorphic towards the plagioclase. The structure of 
this dike-rock is really, so far as I can judge, quite identical 
with that of many crystalloblastic amphibolites. The pave- 
ment-like fabric of the plagioclase mass is the same, so is 
also the idiomorphism or, as one would call it in a meta- 
morphic rock, the idioblasticity, of the mafic constituents, 
and the similarity is still accentuated by a marked parallel 
arrangement of most of the hornblende and biotite crystals. 
The average grain is about 0.3 millimeters in diameter. = 
The crystals of hornblende and biotite are somewhat larger 
than the feldspar grains. All the constituents are perfectly : 
unaltered. This rock has here been called kersantite to 
which it belongs according to the definition of Rosenbusch, 
being a melanocratic dike-rock of the granite-dioritic series 
characterized by the mineral combination plagioclase-biotite. 
The additional hornblende defines it as a horn blen de- 2 
kersantite. 
It may be remarked that in its composition this horn- 
blende-kersantite differs from all the typical kersantites 
whose analyses are quoted in Rosenbusch” text-book ?) in 
being considerably richer in alumina. Mineralogically this 
deviation means a higher proportion of feldspar in the pre- 
sently discussed rock. Chemically it is more closely related 
to many gabbroid rocks and especially to the essexites, but 
in its geological occurrence it is no doubt a typical dike-rock. 
There is still another chemical feature distinctive of the 
rock under consideration: its high proportion of ferrous 
oxide as compared with the amount of magnesia present. 
Generally such a feature is characteristic of granites and other 
1) H. Rosenbusch, »Elemente der Gesteinslehre>», p. 235 (1901). 
22 
