stringers, as judged in the outcrops, appear to contain nothing but natrolite and then play out and disappear.
Going
westward the proportion of neptunite to benitoite increases until the
latter practically disappears before the stringers die out in that
direction.
THE RELATION TO THE SERPENTINE.
It
would be interesting to determine whether the serpentine intrusion lias
had any direct effect on the production of the phenomena described.
General considerations seem to the writer to indicate that it had.
The
most suggestive characters of the deposit in this connection are
perhaps the similarity in nature of the general meta-morphism of the
enclosing rock-lens to the more active alteration in immediate
proximity to the vein on the one hand; and on the other hand the
correspondence of the albitic alteration, the albite stringers of the
country, the albite of the thin zone, and the natrolite in the
benitoite veins which are all related chemically and by field
relations. In other words, the general rock changes which seem to
demand a general elevated temperature for their consummation, appear
genetically related forerunners of the local action along the veins.
Such a general elevation of temperature was undoubtedly supplied by
the peridotite intrusion, and that occasion is most naturally taken as
the period of meta-morphism.
As
regards the more extreme metasomatosis and later mineralization, it
would seem that the rocks of the included mass were more favorable to
the production and maintenance of channels than the surrounding
peridotite, and acted as a vent pipe for the escape of solutions from
the more highly heated lower portions of the peridotite mass to the
overlying rocks, or possibly to the surface.
In
considering the possibility of an origin of the deposit previous to
the peridotite intrusion, the close relationship of the distribution of
the veins and the present form of the included mass is suggestive. The
veins run along in the direction of elongation of the lens and become
smaller, less mineral-bearing and play out just before reaching the
serpentine in either direction.
