[Amr] GEOLOGY OF SOME CITIES IN EASTERN CANADA 155 
series, the lower series consisting of foliated gneisses devoid of lime- 
stones, and generally known as “fundamental gneiss” akin to the 
“ Ottawa gneiss” of Logan. The upper series is generally characterized 
by the presence of crystalline limestones, gneisses, and quartzites, which 
have the chemical composition of ordinary sediments, but are con- 
spicuously banded, to which the term “ Grenville series” or Grenville 
formation is applied. 
Earlier and Later Laurentian.—At Oka, St. Columban, St. Jérome, 
and Ste. Sophie, excellent outcrops of the Lower Laurentian gneisses, 
of the fundamental series, may be studied to advantage, whereas in the 
district south of Rawdon and at St. Sauveur, the crystalline limestones 
and associated rocks of the newer Laurentian or Grenville formation 
have been recorded by Dr. F. D. Adams and Dr. R. W. Ells. 
Besides the above Archean series, at St. Jérome, Lakefield and 
Morin, north of Montreal, masses of anorthosite rock described as “ an 
intrusive rock belonging to the gabbro class, but characterized by a 
great preponderance of plagioclase felspar,” are found to be well de- 
veloped and form important areas. 
Intrusives—The Morin area of anorthosite is one of the largest in 
the world, covering nearly 1,000 square miles. The term “Norian,” 
proposed by Dr. T. Sterry Hunt for these rocks, was for some years held 
to form part of the Laurentian system and constituting its uppermost 
member. These anorthosites are definitely ascertained to form intru- 
sive masses of more recent date than either the Lower and Upper Laur- 
entian above described which they penetrate and affect. They are found 
intruded in the Laurentian of both the upper and lower series, and are 
consequently newer than the Grenville series or formation, but are not 
known anywhere as yet to cut or alter in any way the Ordovician forma- 
tions of the district, and are consequently held to be pre-Potsdam, but 
post-Grenvillian. 
THE ORDOVICIAN SYSTEM. 
The Potsdam formation—As at present developed and exposed, 
through ages of erosion, the Potsdam formation varies in thickness from 
a few feet to upwards of 250 feet and consists for the most part of light 
yellow and rusty-weathering sandstones and conglomerates, usually 
present in the basal beds of the formation, and holding but few remains 
of organic life. These however, at Ste. Anne and on Isle Perrot, consist 
for the most part of the tracks or trails of marine organisms (Protich- 
nites) upon the surfaces or slabs of sandstone upon which ripple marks 
and wind marks are clearly visible indicating that in those remote and 
