|
 Stromatolite
is the oldest of all fossils, and with much labor (cutting and
polishing), it is most beautiful. The banding that commonly appears
in stromatolite is a record of the growth patterns of colonies
of microorganisms, principally photosynthetic prokaryotes (e.g,
cyanobacteria). The colors that are often expressed are the result
of the interaction of biological and sedimentary processes, together
with subsequent chemistry and mineral exchange. Note the contrast
in the polished and unpolished sides in the first two pictures.
This
stromatolite is at once, ancient (Lower Proterozoic), beautiful,
and unusually colored. Coming from the Chocolay Group in an area
know as the Chocolay Hills located southeast of Ishpeming, Michigan,
it dates to some 2.2 to 2.4 Billion years old. This stromatolitic
carbonate has been hand-polished to a mirror finish on one side
exhibits a broad spectrum and intricate pattern of colors.
This
is a rare form from the Northern Michigan locality known as Girvanella
(denoting a form appearing as an individual growth colony of non-symmetrical,
pseudo-spherical shape. Girvanella forms are rarely found in older
stromatolite. Such a massive pseudo-sphere as seen here may have
no precedent in the formation. The portion in the specimen that
appears to be about one half the width colony is nearly a foot
wide, implying a two-foot wide colony. This specimen is also interesting
in that it exhibits both flat and wavy lamination outside the
Girvanella form, and there are red blotches that are precipitated
Rhodochrosite. Measuring
11 by 9.6 inches, such plates are exceedingly difficult to cut
and polish.
Also
see: Stromatolites
Across Geologic Time |
