Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Figure 34. Hand samples of Proterozoic muscovite felsic schist (Xmfs)
showing medium- to coarse-grained, quartz-rich variety (left) and fine- to
medium-grained muscovite porphyroblastic variety (right) .
Xbfg Biotite felsic gneiss (Early Proterozoic) – Biotite felsic gneiss is relatively
common in the large lithologic-structural domain that occurs in the southwestern part of
the Maysville quadrangle. It occurs as the primary lithology in four domains, including
three domains where it is associated with abundant amphibolite gneiss (Xag) and one
domain where it is associated with Berthoud-type granite and pegmatites (YXgp). It is
also present in two domains where amphibolite gneiss is the dominant unit. The biotite
felsic gneiss is commonly mixed with, and locally interlayered with, amphibolite gneiss
(Xag) and hornblende intermediate gneiss (Xhig) and may be transitional with these
units.
Biotite felsic gneiss is light to medium gray, mostly fine grained, and weakly to
moderately foliated. It is a quartzo-feldspathic-rich rock with well-developed, regular
gneissic layering. The biotite felsic gneiss is distinctly finer grained than the muscovite-
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