GENERAL DISCUSSION.
The
Franciscan series extends with various interruptions for about six
hundred miles along the Coast Ranges of California and Oregon and in it
and its associated eruptives are found a very great number of
occurrences of vein deposits that lie among similar surroundings, and a
comparison of these with the deposit now under consideration yields
some general analogies along with the striking differences.
Very
often in traversing the serpentine areas, so frequently associated with
the Franciscan, we come across included masses of partly recrystallized
basic rocks or irregular areas or lenses of glaucophaiie, hornblende,
or other basic schists. These are often cut by veins of quartz, but of
particular interest in the present connection are the abundant veins of
albite. Natrolite, so far as known to the writer, has not been found
under these conditions except at the benitoite locality, but the
analogous albite veins are very common.
Titanium
is often found in the recrystallized rock, occasionally in the veins,
and occurs most commonly as titanite, sometimes as rutile.
While
minerals containing potassium (usually muscovite). magnesium and iron
(such as chlorite or more rarely talc) are also found in veins cutting
such rocks, an association representing so many metallic elements in
essential quantities as occur in the benitoite veins is exceptional:
sodium, potassium, magnesium, iron, manganese, copper, aluminum,
barium. With this variety it seems peculiar that calcium is practically
absent within the veins proper. It is found generally in the veins in
the schists as lawsonite or the amphiboles.
The
occurrence of barium as an essential constituent is without precedent.
The only barium mineral that the writer has found associated with such
formations is barite in veins in the serpentine of Mt. Diablo. While
suggestive of the presence and possible concentration of barium in such
rocks, it is not an analogous occurrence.
Apart from the chemical differences, a comparison of the chief gangue materials, albite and natrolite, would indicate that
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