METAMORPHISM OF THE COUNTRY ROCK. 
123 
to trace the results growing out of another probable premise. If it be 
assumed that during the metamorphic processes the volume of any 
given mass of the rock has remained constant, then the analysis of 
the altered rock may be recalculated on this basis, using for this pur- 
pose the determined specific gravities, and be then compared with 
that of the unaltered rock. The result of this comparison is shown in 
Column 116 
It appears from this total that under the assumption of constant 
volume the rock has lost only about 2 per cent of its substance through 
metasomatism. Moreover, there has been an addition of 8.5 per cent 
of silica and 2.3 per cent of alumina, In other respects the alteration 
is not sign ilicantly different from that deduced upon the hypothesis 
of constant alumina. 
It is, of course, possible that neither the hypothesis of constant 
alumina nor that of constant volume is strictly true; but it is believed 
that the latter is a closer approximation to the truth than the former. 
The altered rock is not notably porous, nor does it, in the held, show 
evidence of contraction in bulk. Moreover, the very obvious intro- 
duction of quartz into some faciesof it and the occurrenceof diaspore 
point to that assumption involving the introduction of silica and 
alumina as being nearest the truth. 
In order to further tesl this point a special examination was made 
of thespring water issuing above the Guston mine (sec p. 114) to deter- 
mine whether it carried alumina in solution. It was found to be 
present to the extent of about 214 parts per million, thus proving that 
the alumina in the rocks can not be regarded as a constant constituent 
during metasomatic alteration in this region. 
The mineralogical composition of the altered lock may be roughly 
calculated from Analysis II as follows, the soda being considered as 
accompanying the potash in sericite: 
Mineralogical composition of metasomatically altered wall rock of Polar Star 
mine. Engineer Mountain. 
Mineral. 
Percent 
age by 
weight. 
Mineral. 
Percent- 
age by 
weight. 
Quartz 
■IH.8 
130.3 
ti.ti 
7.2 
3.9 
1.2 
Kaolinite 
Diaspore 
Pvrite 
98. 7 
Sericite 
The specific gravity of the rock, as derived from the foregoing min- 
eral composition and excluding pore spaces, is about 2.9, as compared 
with 2.8 similarly obtained for the unaltered rock. This rough cal- 
culation is thus in agreement with the general rule that near fissure 
veins the replacing minerals are as a whole denser than those replaced. 1 
Lindgren: Trans. Am. Inst, Min. Eng., Vol. XXX, 1900, p. 594. 
