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ANIMIKIE ROCKS ON THUNDER BAY. ay (sl 
form bold ridges several hundred feet in height, crossing the country in 
straight lines. Dikes like the smaller ones of Pigeon Point also occur, and 
also great overflows and interbedded masses of basaltic rocks. At the first 
great fall, two miles above the mouth of the river, two of these great dikes 
are to be seen. One of these, over which the river falls, is, according to 
Richard Owen,! 212 feet wide, and its course northeast, which corresponds 
with the course of similar great masses noted on the west side of Thunder 
Bay. The rock of this great dike is a medium-grained to coarse-grained, 
light-gray, rough-textured, highly crystalline olivine-gabbro. The thin 
section shows exceedingly fresh and abundant olivine, along with tabular 
anorthite, very fresh diallage, and a little magnetite. 
Following the west shore of Thunder Bay from Pigeon Point, the 
slates with interbedded and overlying masses of fine-grained diabase and 
coarse gabbro, and dikes of the same rocks, are displayed on a grand scale. 
The numerous islands in the mouth of the bay are also composed of the 
‘same rocks, many of the smaller islets showing only the dike rock, a 
whole line of islands marking often the course of a single dike. The shore 
of the bays south of the valley of the Kaministiquia is often overlooked 
by bold cliffs of slate and gabbro, 500 to 800 feet in height. The slates of 
these exposures vary from soft, thin-laminated, black or dark-gray clay- 
slates to hard, ringing, light-gray quartzites occurring in layers several inches 
in thickness. All show a distinctly fragmental character beneath the mi- 
eroscope, but many of the quartzites show also quartz areas like those of 
an ordinary gneiss. The dip of the slates continues to the southeast in this 
distance, the trend growing, however, more and more northerly as the valley 
of the Kaministiquia is approached, while the dip is usually flatter than 10°, 
being often not more than 2° or 3°. 
Of the dikes in this distance one class, including the broader ones, in 
which the rock is relatively coarse-grained, are commonly of a very fresh 
olivine-gabbro like that of the great dike at the falls of Pigeon River, which 
belongs to this class. These larger dikes can be finely seen, for instance, on 
the north side of the south point of Big Trout Bay, where they project into 
the water in great buttress-like forms, 100 to 200 feet in height and trend- 
1 Owen’s Geological Survey of Wisconsin, Iowa, and Minnesota, p. 405. 
