Cherts and Dolomites. 99 
On THE CHERTS AND DOLOMITES OF THE ANIMIKIE 
Rocks oF THUNDER Bay, LAKE SUPERIOR. 
By Exrric Drew INGALL, Ottawa, Ont. 
A number of interesting features are presented by the 
rocks under consideration, but before dealing with their 
nature and structure it will be necessary to give a short 
explanation of their mode of occurrence for the benefit of 
those unfamiliar with the formation. 
The Animikie or silver bearing rocks of Lake Superior 
occur in the vicinity of Thunder Bay. Starting from the 
small exposure of these rocks at the end of the peninsula 
between Thunder and Black Bays they form a narrow 
fringe along the western and northern shores of the former 
which connects with the main area of these rocks extending 
W.S. W. from Port Arthur. This area constitutes a belt 
from twenty to twenty-five miles wide, abutting on the west 
shores of Thunder Bay between Pigeon River and a point 
about eight miles east of Port Arthur and extending in a 
W.S. W. direction across the international boundary into 
the State of Minnesota. Along the northern fringe of the 
formation the rocks are found lying on the smooth surface 
of the Archean rocks, while in a southern direction they 
pass beneath the traps and conglomerates of the Keweena- 
wan, famous for its native copper mines. The dip of the 
whole is at a very low angle to the S.S. E. 
The Animikie formation is made up of traps, argillitcs 
and shales, cherts and dolomitic rocks. The traps present 
themselves as sheets at various horizons in the formation, 
and as more or less vertical dykes cutting the sedimentary 
rocks. The dykes can often be seen to connect with the 
sheets. In many places the sheets preset unmistakeable 
evidence of having been intruded between the bedding 
planes of the sedimentary rocks showing flat under and 
curved upper surfaces. The stratification of the enclosing 
rocks being curved upwares comformably tu the latter. Mr. 
W. S. Bailey of the Johns Hopkins University, Baltimore, 
U.S. A., who examined a number of thin sections of these 
