1898-99. | THE GEOLOGICAL HISTORY OF LAKE SUPERIOR. 49 
similar to those which traverse the Huronian. Geographically these 
eruptive granites are scattered around the lake outside of the area of 
the Cambrian igneous overflows, or in other words they lie, in a general 
way, between this area and the vast region of primitive gneiss beyond. 
Some of these granitic masses are large and are elongated parallel to 
the present coast lines, such as the one called the Giant’s range, lying 
back from the northwest shore in Northern Minnesota and the Thunder 
Bay District, some of those near Nipigon Bay, Otter Head, and thence 
eastward to Michipicoten River, and Pointe Brule, from which a wide 
mass runs inland. 
The Nipigon or Keeweenian formation, consisting largely of the 
igneous rocks already mentioned, occupies the outermost large points 
and islands all around the lake, such as those of Nipigon Bay, Keeweenaw 
Point, Isle Royale, Michipicoten Island, Gargantua and Namainse,* and 
it is probable that the bed of the whole lake consists mostly of this 
formation. The dip of both the Nipigon and the underlying Animikie 
rocks on both sides is towards the centre or deeper parts of the lake. 
The lake-basin existed before these rocks were laid down and it is 
probable that their present dip is partly due to a gentle inward slope of 
the surface on which they were deposited whether as sediments or 
volcanic sheets, but the higher dips which they have now assumed are 
believed to be owing to the slow sinking or caving-in of the crust 
following the removal, from beneath the basin, of such vast quantities 
of igneous matter to form the sheets of greenstones in the Animikie and 
the amygdaloids, etc., of the Nipigon formation, of which only fragments 
remain at the present day. 
Those portions of the shores of Lake Superior which are occupied by 
the Laurentian, Huronian and older Cambrian rocks are noted for 
being traversed by great numbers of dykes of various kinds of green- 
stones. The larger fissures, marked by these dykes, may at one time 
have allowed great quantities of molten matter to escape to form the 
trappean overflows of the Animikie and the amygdaloids, etc., of the 
Nipigon series. On the other hand, these rocks may have been derived 
mainly from volcanic orifices now covered by the lake or represented 
by the plutonic necks to be found among the Huronian strata. 
In the report on my survey of Lake Nipigon in 1869, I have shown 
that the immense horizontal cappings of diorite forming the upper part 
of the Nipigon series around that sheet of water appear to have flowed 
in from the direction of Lake Superior. 
* Namainse, meaning the little sturgeon, is the proper spelling of this name, but it is generally 
incorrectly written, ‘‘ Mamainse.” 
