THE LOWER HURONIAN. 347 



From the data obtained here the total thickness of the slates across this sjn- 

 cline is calculated to be 5,000 feet, which divided in half gives us a maxi- 

 mum thickness here of 2,500 feet. It is impossible to state positively 

 whether or not this represents the maximum thickness of the slates for this 

 area. The presumption is, however, that the maximum thickness here 

 obtained does not represent the maximum original thickness of the slates, 

 as they must have originallj^ occupied the entire basin between the older 

 rocks, and possibly had a much greater vertical extent, the rocks above 

 those now remaining having been of course long since removed by erosion. 



INTERESTING LOCALITIES. 



Structure and relations of the slates. — On Carp and Birch lakes the 

 relations of the topography to the geologic structure are well indicated. 

 The central headland of Carp Lake, as well as much of the western bay 

 and the headlands beside the narrows, are composed largely of a coarse 

 graywacke which is characteristic of the lower part of the slate formation. 

 These headlands are directly along the strike of the plunging anticline at 

 the east end of the lake. Corresponding to them there is an eastward- 

 projecting headland in the west bay of the lake, and this also is composed 

 of the coarse graywacke. The southern part of the western bay is com- 

 posed of the normal slates. Thus, while Carp Lake is as a whole a part 

 of a monocline dipping to the south, it very plainly has a subordinate 

 anticline through the center which makes the graywacke at the base of the 

 slates the predominant rock exposed. 



Birch Lake, west of Carp Lake, was found to be surrounded by slates 

 for about a mile at its eastern end. Then, at a little bay occurring just 

 east of the long westward-projecting point, greenstone comes out upon the 

 north shore for a short distance. The slates appear again iipon the point 

 to the southwest of the greenstone. The greenstone is again exposed in 

 the next bay to the north, and with a minor fold swings up into the 

 extreme north bay and thence inland, the shore being composed of slate. 

 The relations are such as to indicate a series of infolds, the slates occupying 

 the dejiressions, appearing as reentrants on the lakes, and the greenstones 

 occupying the headlands and highlands to the north. 



Topographic depressions usually separate the Archean greenstones 

 from the slates which lie near them, hence the sequence of rocks from the 



