ANIMIKIE ROCKS OF THUNDER BAY. 375 
northeast, is again exposed on the banks of a brook rather more than a 
mile north of the Algoma mine.” The same interest attaches to the oceur- 
rences at numbers of places of layers of concretionary chert and “bands of 
a reddish jasper,” and dolomite, 
Bounding this slate region on the north is a district or belt of granite 
and contorted gneiss. From Kakabika Falls the southern boundary of the 
gneiss is “roughly indicated by a line drawn from the Great Falls to a 
point on the north shore of Thunder Bay, about six miles east of the mouth 
of Current River.” All along this shore of Thunder Bay I found the con- 
ditions observed on the west shore repeated, save that the great bluffs of 
slate and gabbro are wanting. Numerous interbedded layers of black 
crystalline rocks were observed, however, dipping southeast 5° with" the 
accompanying slates, and affected by a strong cross-columnar structure. At 
Bare Point, for instance, three miles below Prince Arthur’s Landing, and a 
mile below the mouth of Current River, there is a thin interstratified bed, 
dipping 6° to 8° southeast,’ of a very fine-grained, dark-gray diabase-por- 
phyrite, with a groundmass composed of tabular plagioclases, augite gran- 
ules, and non-polarizing substance, and abundant porphyritic erystals of 
labradorite, and a few of altered augite. The rock is closely allied to the 
ashbed traps of the Keweenawan. 
East from Goose Point, nearly to the head of Thunder Bay, the older 
eneiss is generally seen at the bottoms of the coves, while the points and 
islands are formed of the slates with interstratified crystalline rocks. A 
ten-foot bed of cross-columnar, light-gray, coarse-grained rock from one 
of the islands nearest below Goose Point proves under the microscope 
to be, as was expected, a very fresh olivine-gabbro, with abundant oli- 
vine, diallage, augite, anorthite, and titanic magnetite as the constituents. 
A twenty-foot columnar layer, seen on some of the islands just opposite 
Caribou Island, and on an adjoining point of the mainland, is also olivinitic 
diabase or gabbro, but finer-grained than the last, and with the olivine 
much altered. Near the head of Thunder Bay some of the cherty layers 
of this series are to be seen. Some of these layers show chert interlami- 
1Not 20° to 25°, as Bell hasit. Op. cit., p. 325. 
