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These milky white, thick, barren quartz veins are clearly of different
origin from the colourless to smoky, commonly mineralized, usually narrow,
networks of quartz veins that cut them and that are attributed to a later,
epigenetic (probably magmatic) origin.

RELATION OF METAMORPHISM TO STRATIGRAPHY AND STRUCTURE

Any explanation of the development of metamorphic changes in the
Tenakihi group rocks must account for both the general correspondence
of metamorphic grade with stratigraphic position over large areas, and
the marked parallelism or near-parallelism of the schistosity and bedding
planes. That the metamorphic changes took place during a time of
differential rock movement is suggested by the observation that the common
minerals found in these rocks are all minerals that can form under condi-
tions of shearing stress—minerals such as andalusite, nepheline, and scapo-
lite, which are unstable under applied stress appear to be missing—and
from the textural evidence that the rocks were being deformed during
recrystallization.

One mechanism that is effective in producing a uniform increase of
temperature and pressure conditions at successively lower stratigraphic
positions over a large area is simple deep burial of relatively undeformed
sediments. In this connection it is of interest to note that the structurally
conformable Ingenika and Tenakihi group beds have a measured total
exposed thickness in Tenakihi Range of approximately 26,400 feet; and the
maximum apparent total thickness of the two groups in the map-area is of
the order of 35,000 feet. The observed sedimentary record thus indicates
that the lowermost Tenakihi beds now exposed were buried at least 6 miles
at the time the uppermost Ingenika sediments were deposited. The stresses
contributing to the metamorphism must, therefore, likewise have been
active at depth and, consequently, regional in character.

The general relations between bedding planes, and cleavage and
schistosity foliation, set forth by Leith (1905) have been accepted by most
workers as explaining the development of schistosity in orogenically
deformed rocks. Under the influence of orogenic compressive forces the flow
cleavage is oriented in the direction of maximum elongation of the deformed
rocks (the direction of the easiest relief of stress), which cannot be parallel
with the bedding of folded rocks except along the limbs of isoclinal folds.
According to these principles, the schistosity and the present folds of the
Tenakihi group rocks cannot have been developed simultaneously by a
tangential compressive force, for the schistosity is parallel with the bedding
in both the limbs and the crests of the major folds.

In view of the fidelity with which metamorphic grade follows strati-
graphic horizons through all parts of the Tenakihi group anticlinoria,
including the overturned limb in the Butler Range, it seems very improba-
ble that the main schistose foliation could have developed during the
folding that produced the present large-scale structures. If the schistosity
is postulated to have developed perpendicular to the direction of greatest
stress, it is very difficult to conceive of a mechanism that would produce
the required change of direction of stress concurrently with the develop-
ment of the present folded structures, for the normal to the plane of
schistosity changes direction as much as 90 degrees in 3 miles across the