9i 



towaning bay. Inland, the firm dolomite of the Cataract 

 formation rises as a low escarpment with irregular outline, 

 and farther south, the Lockport dolomite rises as impressive 

 cliffs ioo to 200 feet (30-60 m.) in height. This remarkable 

 series of escarpments is the result of the wearing back 

 from the Pre-Cambrian oldland of the edges of alternating 

 hard and soft strata. The cliff-forming strata are either 

 limestone or dolomite, and are underlain without exception 

 by soft shales. The land surfaces, excepting the escarp- 

 ments, are fairly level and tend to dip with the formation 

 to the southwest at about 50 feet per mile (15 m. per 1 .6 

 km.). Locally, glacial abrasion and glacial debris in the 

 form of mounds and ridges have tended to confuse the 

 otherwise symmetrical physiographic forms. Sorted, 

 waterworn gravel, found in some localities, indicates the 

 former submergence of all but the highest parts of the 

 island. 



Silurian Section. 



The Silurian of Manitoulin island has been divided 

 into two formations. The lower for which the term 

 Cataract has recently been proposed, consists of 50 to 60 

 feet (15-18 m.) of dolomite overlain by 27 to 66 feet 

 (8.2 to 20. 1 m.) of red clayey shale. The upper or Lock- 

 port (Niagara) formation consists of at least 240 feet 

 (73 m.) of dolomite. 



Cataract formation. — The Cataract strata rest with 

 apparent conformability upon the green shales at the top 

 of the Richmond formation. The dolomite near the base 

 is thin-bedded and argillaceous; midway up, thick beds 

 of massive dolomite occur; and thin beds are again present 

 near the top. Bryozoan and coral reefs, several yards 

 in diameter, frequently occur within the upper 20 feet 

 (6 m.) of the formation, and appear to have caused local 

 thickening of the dolomite. In the lower shaly argilla- 

 ceous dolomite, ramose bryozoans are plentiful and about 

 10 feet (3 m.) above the base Leptcena rhomboidalis has 

 been sparingly found. The characteristic fossils of the 

 formation occur mostly in the upper beds and are: — 



Clathrodictyon vesiculosum Nicholson and Murie. 

 Acervularia gracilis {Billings). 



