33 



distance to the north and northeast the limestones of the 

 Black River and Lowville formations, which underlie the 

 Trenton limestone and rest unconformably on the Pre- 

 Cambrian rocks, are well exposed and overlap the Pre- 

 Cambrian. The northern end of Lake Couchiching is 

 occupied by the Pre-Cambrian rocks, and near the contact 

 an escarpment is generally developed in the limestone, 

 which affords numerous sections. 



Section of the Lowville Formation. 



At the Longford quarries on the west side of Lake 

 St. John, about 8 miles (12-9 km.) northeast of Orillia, a 

 good section is exposed of Lowville (Birdseye) limestone 

 with basal series of shales and sandstone or arkose. The 

 eastern and northern sides of the lake are occupied by 

 Pre-Cambrian rocks, but along the western side a limestone 

 escarpment is developed, in the face of which a number of 

 quarries have been opened. The beds dip slightly to- 

 wards the southwest, and at the north end of the lake over- 

 lap the crystalline rocks. Near the contact the beds have 

 a steep dip and appear to be faulted. 



The general section of the Lowville formation is as 

 follows : — 



1. Basal series of sandstones, shales, etc. — The 

 base of the series consists of a few feet of coarse, calcareous 

 sandstone or arkose, which rest unconformably on the 

 Pre-Cambrian crystalline rocks. These beds pass upward 

 into red and green shales with intercalated lenses or thin 

 beds of sandstone, and occasionally thin beds of fine- 

 grained, dove-coloured limestone. The thickness of the 

 series varies, and the beds are frequently absent on the 

 sides and tops of ridges or domes of the crystalline rocks, 

 where the limestones are seen to rest directly on the old 

 floor. The sandstone and shales are best developed in 

 basins between ridges of the crystalline rocks, where they 

 occasionally have a maximum thickness of about 40 feet 

 (12 m.). They are local in character and derivation, and 

 evidently represent the old soil covering of the Pre-Cam- 

 brian rocks somewhat sorted, rearranged and recemented, 

 and it seems probable that they represent the initial near- 

 shore deposit of the next succeeding formation. 



35066—3 



