Lo):

The original rocks in the Cache Creek series could not by simple
weathering have been turned to clay, since the clay contains elements
-which have been added to them presumably by hot ascending solutions.
The residual clays in the Lower Cache Creek series occur in rocks com-
posed essentially of quartz in very fine grains, in some cases 005 millimetres
and less, with a small percentage of muscovite or hydromica. Where’ the
rocks have disintegrated to clay the change appears to have consisted solely
in an increase in the amount of hydromica or in other cases of hydromica
with kaolin and other clay-forming substances. The residual clays in the
Lower Tertiary Volcanics lie in lavas and ash rocks that have glassy or
very fine groundmasses, and the change to clay has produced what looks
like hydromica, kaolin, iron oxide, carbonate, et¢. In this instance the rock
itself possibly could have furnished the elements for its alteration products.
In residual clays in Cache Creek rocks, however, the fresh rock is made up
essentially of quartz which cannot, by the agency of atmospheric waters,
be turned to clay-forming elements, nor is there enough hydromica in the
parent rock to account for the clay by simple alteration of that mineral to
kaolinite. The increase of hydromica and the actual replacement of quartz
by it can be accomplished only by the agency of hot ascending solutions.
™ places like the Bonaparte clay belt, the widespread occurrence of pyrite
in the parent rock shows that there has been mineral deposition through
the agency of ascending solutions. The presence of gypsum in these clay
banks shows, on the other hand, that oxygenated atmospheric waters have
also had an effect, forming sulphate waters from the pyrite after its deposi-
tion, and from these gypsum, CaSO.,+2H.0, has been precipitated.

The zone of decomposition is related to fracture zones rather than to
a land surface or zone of weathering. In all cases where residual clays were
examined the rocks in which they occur have been much disturbed, that is,
either closely folded or faulted and brecciated, or both crumpled and
faulted. Figure 11 illustrates the close relationship at Chimney Creek
bridge between a zone of faulting and the extent of the residual clay. At
17 Mile ranch, the Lower Voleanics are lying in a syncline and it is along
the upturned beds on the edge of the syncline, where a great deal of fault-
ing has taken place, that the clay banks lie (Plate XII). On the Bonaparte
(Plate XIII) the Cache Creek beds are also much disturbed and similar
beds in Baker canyon have been much crumpled. In all these places it is
possible to find beds of the original, unaltered rock lying at the same level
as the altered material. At Chimney Creek for example, fresh rock not
only underlies the clay belt but is found at the same level to the north of it.
At Baker canyon there is fresh rock over the clay.

Age of Formation.

The absolutely unaltered. character of the Miocene basalt lying

directly over Cache Creek clay in Baker canyon, proves that the clay-form-_

ing processes had ceased to act in the late Miocene. In Baker Creek
canyon, near the occurcence of residual clays in the Lower Cache Creek
rocks, masses of the same residual clay occur as boulders in a gravel
carrying plant remains and underlying the upper Miocene basalts. This
gravel belongs to the Fraser River formation and the clays were, therefore,
formed in the Cache Creek rocks before the Fraser River beds were laid
down. The Lower Volcanics of Miocene age that underlie the Fraser River

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AOA ae A SE StI