304 R. BELL — HONEYCOMBED LIMESTOISES IN LAKE HURON. 



eopyrite comes to the surface, the presence of acid sulphate could be 

 detected by the taste of the water, and when the latter, containing as it 

 does vegetable matter also, was used experimentally for the boilers, it gave 

 off a most offensive smell and had a very corrosive effect. The streams 

 of the whole of this Huronian region no doubt receive many contribu- 

 tions of acidulated water from similar sources. As the decomposition of 

 the greenstones and other pyritiferous rocks, and also the oxidation of 

 the drift materials derived from them, proceeds, the quantity of acid 

 derived from them and carried into the northern part of lake Huron 

 will increase. At The Narrows between Cloche peninsula and Little 

 Cloche island, as already mentioned, the pits in the soft argillaceous 

 grey limestone are surrounded by a zone stained by iron oxide. This is 

 just what might be expected to result if such a rock were being slowly 

 dissolved by iron-sulphate and, therefore, this fact helps to support the 

 present hypothesis. 



The fresh rock-surfaces and unoxidized drift left at the close of the 

 Glacial epoch would produce a much smaller proportion of sulphuric 

 acid in a given time, and we therefore suppose that the erosion of the 

 limestones in the bottom of the northern part of lake Huron went on 

 even more slowly then than now. 



Another example may be cited here of water containing sulphuric 

 acid which has been derived apparently from volcanic rocks. A sample 

 from the shallow fresh-water in the estuary of the Nelson river, Hudson 

 bay, was taken by the writer to the late Professor William Dittmar, the 

 well known authority on water analyses, and he found it to contain no 

 less than 4.73 grains of sulphuric acid to the imperial gallon.* The 

 source of this acid appeared to be the drift-material which had come 

 from the volcanic rocks of the central or eastern part of the bay. 



Conclusions. 



The conditions which have contributed to the production of the pecu- 

 liar forms of erosion above described appear to have been : 



1. The internal structure of the limestone itself. 



2. A small quantity of acid in the water acting for a great length of 

 time. 



3. A considerable depth of water, the hydrostatic pressure seeming to 

 promote the dissolving of the rock. 



4. Freedom from sediment during the long time required. 



5. The rock must be exposed to the open or free action of the water. 



6. Shifting currents in the water would also appear to assist the process. 



*See Appendix V, Report C, of the Canadian Geological Survey for 1879-'80, p. 78, 



