130 ROYAL SOCIETY OF CANADA 
Ottawa is situated close to the boundary line between the great 
Archean complex, and the flat Ordovician plain to the south. The 
Laurentide Hills to the north of Ottawa city belong to the oldest system 
we know of on this planet, and the bluffs and promontories or points 
which mark the right bank of the Ottawa river so conspicuously and the 
country to the south, belong to the marine sediments of the old Ordovi- 
clan sea : the one was deposited at a time when the earth was in an 
igneous condition, whereas the latter, which constitute the bedded rocks 
of this district, were deposited in salt water, and are therefore of aqueous 
origin. 
LAURENTIAN and HURONIAN. 
Under this head are classed those intricate rock formations which 
consist of foliated, garnetiferous, rusty and granitoid gneisses, pegma- 
tites, Jasper, garnet, and associated minerals. 
According to the most recent divisions made in Archean rocks of ~ 
this portion of North America, by Dr. F. D. Adams, Dr. A. E. Barlow, 
Dr. R. W. Ells, and other Canadian geologists, the “ fundamental 
gneisses,” which occur more or less sparsely distributed in the Lauren- 
tide Hills along the north side of the Ottawa river, to which the term 
“Ottawa gneiss” has been applied, are held to constitute the Lauren- 
tian proper in its restricted sense. Overlying these gneisses, there 
occur bands of crystalline limestones, crystalline dolomites, ophi-calcite, 
sillimanite gneiss, garnetiferous gneiss, which are referred to an upper 
division, the Grenville series (Grenvillian of Sir Wm. Dawson), and by 
some now held to be possibly the equivalent of Huronian rocks in their 
type locality and region north of Lake Huron. 
The total thickness of the various bands of crystalline limestone or 
of gneisses and other areas in this complex cannot be accurately deter- 
mined, but they constitute a complex or net-work of igneous masses for 
the most part the result of magmatic differentiation, in which are im- 
bedded greatly altered rock masses which may have been deposited in 
part, at least, under aqueous influences. These Archean rocks form an 
important mineral belt which has only begun to be developed. 
Mica, appatite, graphite, asbestus, felspar, dolomite, baryte, mag- 
netite and other ores of iron, galena, gold, and marble form the leading 
materials which have been extracted or discovered in the rocks of this 
period of economic value to man, and as this great mineral-bearing dis- 
trict is opened up and developed new and hitherto unrecorded minerals 
will also be found to add to the natural wealth of the Ottawa district. 

