213 SILVER LOCATIONS OF THUNDER BAT. 



NOTES ON THE SILVER LOCATIONS OF THUNDER BAY. 



BY E. J. CHAPMAN, LL.D., 



PROFESSOR OF MINERALOGY AND GEOLOGY, UNIVERSITY COLLEGE, TORONTO. 



Much interest having been excited, within the last few years, by 

 the discovery of rich silver deposits in the district around Thunder 

 Bay, Lake Superior, a brief account of the so-called silver locations of 

 that region may not be unacceptable to our readers. 



1. General Geology of the District : — The strata of the north shore 

 of Lake Superior belong to three distinct periods of formation. The 

 lowest of these strata in geological position, and consequently the oldest, 

 consist of highly crystalline gneissoid beds of Laurentian age. These 

 (marked A, in Figure 2) form the high land which lies, as a general 

 rule, a few miles inland from the lake-margin ; although in many 

 places the Laurentian strata come down bodily to the edge of the lake, 

 or throw off spurs which approach the shore-line more or less closely. 

 The second series of Lake Superior strata (marked B in the diagram- 

 section, Figure 2) consist of green and gray slates and conglomerates; 

 with interstratified beds of quartz-rock, &c., belonging to the Iluronian 

 group. These Huronian strata, however, occur only here and there, as, 

 more especially, at Michipicoten Harbour, Otter Head, Pic River, and 

 along the back of Thunder Bay. Commonly, therefore, the strata of 

 the third series (marked G in the diagram. Figure 2) immediately 

 overlie the gneiss or other crystalline rocks of the Laurentian series. 

 These higher strata form Sir William Logan's division of the " Upper 

 Copper-bearing Rocks of Lake Superior.'' They belong, in themselves, 

 to two probably distinct epochs, although conventionally referred to the 

 horizon of the Caleiferous Series of the east. They are thus regarded 

 as altered strata of Lower Silurian age. They occupy most of the lower 

 lands intervening between the elevated gneissoid region and the shore 

 of the lake. Sir William Logan has subdivided these strata into two 

 series : a lower, lying mostly west of Thunder Cape ; and an upper 

 series, ranging to the east of that landmark. Both are traversed by 

 numerous dykes and masses of eruptive trap or greenstone ; but, whilst 

 the trappean rocks associated with the lower series are more or less 

 compact in texture, and exhibit a marked tendency to assume a 

 columnar or sub-columnar structure, those of the upper series are 

 almost invariably amygdaloidal, and they rarely present a columnar 

 aspect. The columnar condition of the lower series arises essentially 

 rom the presence of joints or partings at right angles to each other, 



