86

CHAPTER. Y.
PETROGRAPHY.

INTRODUCTION.

In this chapter will be found detailed descriptions, both
megascopic and microscopic, of the typical rocks found in each
formation. Space limits forbid the description of each of the
hundreds of specimens collected, nor would it serve any useful
purpose. Instead, the specimens have been divided into groups
under each formation, and the group is described as a whole.

MAUDE FORMATION.

Quarizitic Rocks. These are pale greenish and bluish light
grey, very fine-grained, compact, arenaceous rocks, occurring
in flaggy layers up to a foot thick, with a blocky fracture.

Under the microscope they are seen to be excessively fine-
grained rocks, and much of the material is not clearly resolved.
They are characterized by quartz in irregular grains, sometimes
rounded, up to 0-1 mm., and also fewer grains of plagioclase.
Subordinate minerals are recrystallized muscovite, together
with granular calcite and matted chlorite. The texture is finely
even granular, and the minerals are in part recrystallized. Altera-
tion consists of recrystallization of argillaceous material to
muscovite and chlorite, with later introduction and replacement
by calcite.

Calcareous Feldspaihic Argillites. Rocks best classed under
this name form much of the lower part of the formation. They
are well laminated, fine-grained to dense, thinly banded sedi-
ments, nearly always black or dark grey, the adjacent layers
often presenting a striking appearance in weathered exposures,
owing to the brilliant yellow, orange, and black colours of
alternating bands. This type is referred to locally as ‘‘the
ribbon rocks.”” These rocks are frequently highly fossiliferous,
and specimens from virtually every outcrop emit a strong