ACID WATER AND ITS EFFECTS. 303 



The solvent action of the slightly acidulated water ii^, no doubt, aided by 

 certain conditions favorable to it in the present case, but whicli might be 

 absent elsewhere, even if the rcciuisite amount of acidit}^ existed. The 

 water of this part of lake Huron is perfectly limpid and free from sus- 

 pended impurities, so tliat there is nothing to check the progress of the 

 dissolving jn-ocess, no matter how slowly it may be going on. That it has 

 been exceedingly slow in its operation is shown by the pitted or honey- 

 combed surfaces retaining their glaciated forms. As the basin of lake 

 Huron has no doubt been filled with water since the disappearance of the 

 ice-sheet, the time required for the erosion under consideration must have 

 been many thousands of years, but the proportion of acid in the water 

 probably increased gradually, as the explanation to follow will show. 



Sulphuric Acid in the Water and its Source. 



Although the water of this part of lake Huron has not been analyzed, 

 it appears to contain a notable proportion of sulphuric acid. It has the 

 property of slowly reddening vegetable matter, purple or blue. Its cor- 

 roding action on tin pails and pans is a source of annoyance and loss to 

 housekeepers and campers who use the water for domestic purposes. A 

 new tin pail, if kept filled with the lake-water, will in a few days show 

 rusty excrescences in the bottom, which increase rapidl}^ and soon per- 

 forate the vessel. The water of the portion of the lake where honeycomb- 

 ing of limestones is most obvious is distinctly harder than elsewhere, 

 prol>ably owing to the pressure of sulphate of lime. 



The source of the sulphuric acid may be looked for in the Huronian 

 rocks lying to the northward of the lake. It is remarkable that the por- 

 tion of the lake in which this form of erosion of limestone takes place 

 lies directly in front of that part of the north shore occupied by the 

 Huronian rocks, and receives several considerable rivers which drain an 

 immense area of rocks of the same age. The}'- are mostly of volcanic 

 origin and arc rich in sulphides, whereas all the other rocks around the 

 lake are comparatively free from them. 



Sulphides of several metals, but especially of iron, are disseminated 

 tlirough most of the Huronian rocks. Pyrites and i)yrrhotite are par- 

 ticularl}' abundant in the greenstones, and these form a considerable 

 proportion of the series. The two sulphides referred to occur both as 

 disseminated grains and in the form of numerous masses, some (piite 

 large, such as those around Sudbury, a few of which are l)eing worked 

 for nickel. The decomposition of these sulphides upon the surface of 

 the rocks and along their fissures and joints ])roduces in)n-suli)]iate, 

 which is carried away l)y the streams, where its presence is froriucntly 

 shown ])y the precipitation of a part of tlic iron. In the swanij) at tlie 

 >rurray mine near Sud])ury, where a large mass of i)yrrhotitL' and chal- 



