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recrystallized and selectively reconstituted to a strong, hard, gneissic rock
composed largely of biotite and feldspar. The bedding of the tuffs is made
more conspicuous by a relative coarsening of the coarser beds, and minor
features such as graded bedding and crossbedding are emphasized. Nearer
the contact, this process becomes more pronounced, and although the
bedding is still well preserved, individual beds approach the composition
and texture of a biotite diorite. In all cases it is the coarsest tuffs that
are the most profoundly affected; some of the very fine-grained tuffs and
porphyritic flows are not noticeably altered. Where selective reconstitu-
tion is well advanced thin seams and sill-like bodies of white feldspar
appear here and there between bedding planes; most of these are nodular
but very persistent, and seams averaging less than 4 inch thick may be
followed along the same bedding plane for more than 100 feet. These
sill-like bodies become more numerous nearer the ultramafic body, to
where they may constitute as much as one-quarter the rock volume. In
many of the highly recrystallized tuffs it is difficult to distinguish some
‘pseudo-diorite’ recrystallized tuff beds from the injected feldspathic
material, but the latter do not show graded bedding, and numerous dyke
‘feeders’ that connect the sill-like masses appear to demonstrate conclu-
sively that the tabular feldspathic bodies are injected, even though they
are of nearly the same composition as the beds between which they lie.
Where they crosscut the bedding, the dykes have in places produced a zone
of finer grain size, in appearance much like a chilled zone, but with reverse
structural relations, in the ‘pseudo-diorite’ recrystallized intruded rocks.
The dykes themselves appear to be unchanged at their contacts. The
feldspar of many of these bodies is coarse grained, and remarkably resistant
to weathering, so that on many outcrops the interstitial material has been
etched away, leaving a crust of coarse anhedral grains standing in relief.

This feldspar, together with that of the ‘pseudo-diorite’ and recrystal-
lized tufis, is, despite its fresh outward appearance, so highly altered that
its composition is not determinable. Most of the feldspathic material has
been changed to a white, nearly opaque, clay-like aggregate in which the
original feldspar grains cannot be distinguished in thin section. Accom-
panying the clay-like material is much clinozoisite and epidote, mostly
very fine grained and more or less evenly disseminated, but some rocks
contain irregular masses 1 mm. or more in diameter. A few thin sections
of this material contain zoisite. The fragments of feldspar grains tested
have a relatively low refractive index (about 1-540 to 1-542) suggesting,
along with the general mineralogical association, a sodium-rich plagio-
clase, but all feldspars are so altered that the measurements are not
reliable. It is not known whether albitization of an originally more calcic
feldspar has been part of the alteration process.

The injected feldspathic material is relatively mobile, and not entirely
confined to the tuffs; a few small bodies are found in the metamorphosed
siltstones, where, however, it does not form regular sill-like masses, but
thin irregular bodies that wander erratically across the bedding in a manner
suggestive of replacement. Similar material is found sparingly in the
serpentinized ultramafic rock as much as a mile from the exposed contact