CHARACTEKISTICS, AGE AND ATTITUDE. 299 



This variety of deeply pitted surface resembles limestone which has been 

 thoroughly riddled by the burrows of Saxicava. In a well marked case, 

 to be again noticed, where a submerged shelf or hanging ledge was eroded 

 on the under side, the pitting took the form here described. It is well 

 shown in figures 1 and 2, plate 14. 



Age AND Attitude of the eroded Rocks. 



The rocks of Grand l\ranitoulin and the adjacent islands embrace eight 

 ditlerent formations, from the Chazy, in the northern part of the La Cloche 

 island, to the Guelph, on Fitzwilliam island and on the southeastern part 

 of the main island, while dolomites of the Huronian system are met with 

 on many of the islands of the channel between the Manitoulin chain 

 and the main north shore of lake Huron. The Chazy is represented by 

 brownish red and green marls and fine grained white sandstone ; the 

 Black River consists of pure limestone and yellow weathering dolomites ; 

 the Trenton principally of bluish gray limestone, with earthy beds ; the 

 Utica of black bituminous shale; the Hudson River of marls, with thin 

 beds of limestone and sandstone; the Clinton of dolomite, with bright 

 red and green marls at the base, while the Niagara and Guelph forma- 

 tions are composed almost entirely of dolomites. The dip is uniformly 

 to the soutli, at a very low angle, and the naked beds of the higher forma- 

 tions above enumerated slope gently under the lake along the southern 

 sides of all the islands of the Manitoulin group. 



P^ROsioN Forms in Relation to Variety of Rock. 



The largest and most irregular cavities are in the magnesian lime- 

 stones of the Guelph formation. Their appearance in sUa is shown in 

 plate 15, which is from a photograph. The globular and pear-sha})ed 

 varieties, shown in figures 1 and 2, plate 13, are excavated in somewhat 

 argillaceous dolomites of the Niagara and Clinton formations. The cup- 

 shaped hollows are commonest in the pure limestones of the Hudson 

 River, Trenton and Black River and in the dolomites of the last men- 

 tioned formation. The finger-shaped honeycoml)ed structure was found 

 principally in tlie pure limestone of the Black River formation, while the 

 smoother and rather larger excavations are characteristic of the Huronian 

 dolomites. The shallower varieties of this form of pitting reseml)le that 

 of a well eroded aerolite. The occurrence of these various forms of 

 honeycombing and pitting in such a variety of limestones and dolomites 

 in this portion of lake Huron proves that the phenomenon is not due to 

 anytliing unusual in the general composition or to any chemical i)ecu- 

 liarity of a particular variety of rock, but to some outside cause. The 

 various forms which the erosion takes, however; show a slightly unequal 

 solubility connected in some way with th*; int(3ni:il structiirLM)f tlic rock; 



