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minerals (probably mainly pyroxene) are completely altered to antigorite.
The groundmass is very fresh, with minute laths of feldspar arranged in
conspicuous flow lines, curving around the phenocrysts.

Tuffs

Most of the rocks here called tuffs are undoubtedly water-lain, and
some of the material may be a product of erosion of nearly contemporane-
ous volcanic flows, rather than of strictly pyroclastic origin. They are
composed of angular but well-sorted fragments and broken crystals
representative of all the types of flows with which they are interbedded.
In many rocks the degree of heterogeneity within a single bed is consider-
able; fragments of various rock textures and mineralogical combinations
(all apparently within the andesite range) are intimately mixed in the
finest lamine. Other rocks are composed almost entirely of individual
crystals and fragments of crystals of feldspar in a very fine-grained,
uniform matrix. Some may be true crystal tuffs, formed predominantly of
crystals blown out from volcanic vents as separate individuals. Unless
they are well bedded, some of these rocks cannot be distinguished in hand
specimens or under the microscope from porphyritic flows. One distinctive
rock of this type is buff to greenish grey, with a curious spotted appear-
ance due to an abundance of fresh, idiomorphic, doubly terminated prisms
of oligoclase-andesine, about the size and shape of grains of rice, in a
uniform, very fine-grained, chlorite-sericite-feldspar matrix. In a few
places, immediately underneath beds of limestone, highly hematitic, siliceous
material forms the matrix and coats the fragments of the tuff, producing
a brilliant red jasper-like rock.

The individual layers of the banded tuff range from a fraction of an
inch to a foot or more in thickness. Most layers show a pronounced
gradation from coarse to fine grain. At one carefully checked locality on
the ridges west of Swannell River, this gradation is repeated about ten
times per foot through a stratigraphic thickness of more than 450 feet.

Greywackes

With the appearance of fragments of non-voleanic origin, and the
development of a recognizable sedimentary groundmass, the tuffs grade
into greywackes. Except for a few beds rich in carbonate of organic
origin, the proportion of non-voleanic material in these rocks rarely
exceeds 25 per cent, and there does not appear to be a gradation from the
predominantly volcanic greywackes into the arkosic sandstones and grits.
Most of the greywackes are somewhat coarser in texture, and much less
well sorted, than the tuffs. In many beds, fragments up to 4 inch in
diameter are abundant, and subangular pieces of rock up to 1 inch long
are not uncommon. The non-voleanic material consists of fragments of
black argillite, rounded grains of quartz, flakes of muscovite and quartz-
mica rock, and carbonate material, much of which is in the form of disks
of crinoid stems and fragments of bryozoa. The groundmass of some
greywackes is largely crystalline carbonate; of others, it is a pasty, partly
opaque, exceedingly fine-grained aggregate of indeterminate composition.