The Mt. Mary area (S. lat. 42° 12', long. 145°50')
is underlain almost entirely by rocks of Pre-Cambrian
age, the exception being a small downfaulted
block of Crotty (?) Sandstone, (Silurian). The
Pre-Cambrian rocks are divided into three main
groups. The oldest is the Joyce Group which consists
of mica-garnet schists, quartzites and amphibolites.
This is overlain unconformably by quartz
schists, massive quartzites and phyllites of the Mary
Group. Above these are quartzites. mica schists,
garnet-mica schists and amphibolites of the Franklin
Group. Igneous rocks are recognized as premetamorphic
amphibolites (Older Basic Igneous
Group) and post-metamorphic dolerites (Younger
Basic Igneous Group).
The sediments are broadly folded into the Mary
Anticline. Tight, recumbent drag folds are abundant
and are shown by all formations. Cleavage or
schistosity, generally parallel to the bedding, is well
developed in phyllites and schists. A regular,
regional lineation plunges fiatly to the west and is
cut by a sporadic lineation almost at right angles.
Faulting is very abundant and makes the interpretation
of the structure very difficult. Three sets
of faults trend (in order of importance) at 120°,
10° and 60°-80°. The major faults are high angle
reverse faults.
There is some degree of structural control of the
physiography. All rivers are superimposed and two
streams show maturity in their central reaches due
to resistant quartzites forming local base levels.
There is evidence of rejuvenation of some rivers.
Rocks of the Joyce and Franklin Groups are
metamorphosed to garnet grade while the Mary
Group is less altered. Selective metamorphism is
demonstrated and is probably due to the favourable
chemical and physical nature of the Joyce and
Franklin Groups
