160 © NOTICES OF THE MEETINGS [March 19, 
The trap dykes, traversing granites and other crystalline rocks 
indifferently are a singular feature on the north shore and abound 
chiefly from Written Rocks to the bottom of Michipicoton Bay. 
By their dark and undeviating course through the grey, red, or 
green rocks of the rugged coast, they strike the eye of the most 
incurious— if only as ruined staircases, crossing bays and headlands 
and climbing hills for miles. Their size, number, and direction are 
irregular. They may be solitary, or twenty in company—some- 
times all parallel and close together. They often run with the 
general trend of the coast.* 
Mr. Logan divides them into three varieties, according as they 
are homogeneous, sienitic or porphyritic. 
Professor Agassiz distributes the dykes of the whole lake into six 
systems —each with its own mineral character and direction — its 
own epoch of upheaval; and each he announces to have been an 
important agent in giving shape and direction to the district in 
which it occurs. He truly says that the general outline of the lake 
is the combined effect of many minor geological events taking place 
at different periods. With some truth in it, this theory does aot 
seem to take into sufficient account the pre-existing metamorphic 
and granitic rocks, and it overlooks the variety observed in the 
directions of the dykes in the same neighbourhood. 
Dr. B. stated that if he might be allowed to hazard an opinion, it 
would be, that this curious assemblage of dykes— abounding as 
much in the S. as on the N. coast—pervading all the crystalline 
rocks indiscriminately, had ascended independently from the unseen, 
distant, mass of trap beneath. They appear in many ways peculiar, 
and have no visible connexion with the traps he had been describing. 
Before the emergence of either traps or granites, Lake Superior 
received its great outlines from the metamorphic rocks,—thrown 
into their present position by still earlier upward movements: for 
on the eastern half of both shores of the lake they strike E. and W. 
with little variation, while on the western half, these far extending 
rock-masses strike WSW. and SW.—giving thus, to the lake, 
a general eastward direction, with a gentle curve to the north, as 
stated before. This done, Cambrian Sandstone slowly took posses- 
sion of the trough of the lake—just as we see a certain shell marl 
is doing now. ‘The anticlinal Granites, which appeared afterwards, 
only concurred in the same effects;—shaping and elevating the 
adjacent lands. 
In after-geological times important modifications arose in the 
form of the lake. Promontories were pushed out, and islands 
raised up by successive outbursts and overflows of trap from separate 
fissures of great length — those for example of Keweenaw, Thunder 
Mountain, and Isle Royale —all intercalated with conglomerates, 
* Vide Quart. Journal of Roy. Inst. Vol. XVIII. p. 244. Bigsby on Lake 
Superior. 
