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their less-crystalline phases are partly oolitic, and possibly partly algal,
and which in many places contain considerable detrital non-carbonate
material. All of these sediments suggest shallow-water conditions of
deposition throughout the entire sequence of the group. Near-shore condi-
tions are required by the poorly sorted subgreywackes, by the conglomerates
and the relatively coarse sandstones, and by the detrital limestones.
The crossbedded quartzite, and the possibly mud-cracked limestone
indicate shoreline conditions of deposition. If the nitre in Tenakihi Range
and the calcium sulphate (gypsum or anhydrite) found in many localities
are original constituents of the rocks, their source beds may be the result of
isolation and dessication of local basins in an arid climate. The remarkable
purity and high degree of rounding and sorting shown by some of the
quartzites suggest that they may be reworked wind-blown material. Except
for the rare, fossiliferous limestones, which are marine, there is little
evidence as to whether the assemblage was laid down in salt or fresh
water; but the thickness of the deposit and its uniformity over a large
area suggest a sea-coast rather than an inland-basin environment,

The nature of the sediments indicates a greater diversity of conditions
of source area than of deposition. Greywackes and subgreywackes
are characteristically products of rapid mechanical erosion and short
transportation. These sediments, therefore, suggest a near-shore proven-
ance of high relief in which mechanical erosion is dominant; perhaps a
rugged archipelago or mountainous coast. The well-rounded, well-sorted,
fine conglomerates may have travelled far, but probably require transporta-
tion by streams with a relativly high gradient, as from a highland area.
The pure quartzites, on the other hand, if not of desert origin, are probably
indicative of prolonged erosion in an area of moderate to low relief, with
jong transportation resulting in reworking, resorting, and rounding of
particles. The pure, massive limestones must have been formed during a
temporary cessation of clastic deposition over considerable areas.

The intimate intercalation of the various rock types indicates a
repetitive alternation of character and quantity of supplied material and
of conditions of deposition. It is probable that, in a heterogenous sedi-
mentary sequence like the Ingenika group, there are many intervals of
non-deposition in any one section; but no instances of erosion of one
bed before the overlying one was deposited were recognized, with the
possible exception of ‘slump’ structures and fillings of contorted argillite
in irregularities in some conglomerate beds in the Tenakihi Range near the
nitre-bearing beds.

Thus, on the whole, the Ingenika group rocks were laid down under
typical geosynclinal conditions of deposition, in which the sinking of the
sedimentary basin approximately kept pace with its filling by sediments,
through the deposition of an aggregate of up to 18,000 feet of material.

Evidence regarding the lithological character of the source area for
the Ingenika group rocks is meagre. On the whole, the rocks are consider-
ably less siliceous than those of the Tenakihi group. The only sedimentary
rocks whose bulk composition can be considered at all representative of
their parent rocks are the greywackes and subgreywackes. These show,
along with the ubiquitous quartz, a relatively high proportion of ferromag-
nesian material, an abundance of intermediate to slightly sodie plagioclase,