All the varieties hitherto
examined, with the exception of the peculiar one from the Andes, are in
composition and physical characters true opals. This is the case with all
the Indian and Java varieties. They consist essentially of silica in its
colloidal form, the water, lime, potash, and organic matter being as
small and variable in amount as in the mineral opals; and, as in them,
these substances must be regarded merely as mechanical impurities.
The tabasheers must be studied in their relations on the one hand with
certain varieties of the natural semi-opals, hydrophanes, beekites, and
floatstones, some of which they closely resemble in their physical
characters, and on the other hand with specimens of artificially
deposited colloid silica formed under different conditions. Prof. Church,
who has so successfully studied the beekites, informs me that some of
those remarkable bodies present singular points of analogy with
tabasheer.
By the study of thin sections I have, during several years, been
endeavoring to trace the minute structure of some of these substances. In
no class of materials is it more necessary to guard one's self against
errors of observation arising from changes induced in the substance
during the operations which are necessary to the preparation of
transparent sections of hard substances.
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