302 R. BELL — HONEYCOMBED LIMESTONES IN LAKE HURON. 



had been due to this cause, their occurrence would not be confined to the 

 limestones of lake Huron, but would be a general phenomenon in con- 

 nection with similar rocks all over the world. On the contrary, the 

 pebble theory is not sustained by facts anywhere. Pebbles washed by 

 currents passing over rocks or grated against them by the waves, not only 

 do not pit or honeycomb rocks, but they have the opposite effect and 

 wear them smooth. 



It is true that gravel may now be seen scattered over local pitted sur- 

 faces which have been laid bare by the recession of the lake, but on 

 examining cases of this kind it is always manifest that the gravel was 

 placed upon such surfaces long after the completion of the pitting. 



The occurrence of. the cavities equally on the tops of loose blocks of 

 limestone and on the solid beds is another objection to this theory. 



A final objection is the fact that some of the most beautiful examples 

 of deep pitting are found on the under surfaces of overhanging beds, the 

 eroding agent having worked upward in a free body of water. 



Acid Water the probable Cause of the Erosion. 



Having eliminated all the possible causes which have suggested them- 

 selves as being unlikely to have produced the erosion under discussion, 

 the question arises : To what must we attribute it ? It appears'most prob- 

 able that the cause will be found in the differential solubility of the rock 

 in the water of the lake itself The slight difference in the solubility of 

 those parts of the rock which give rise to the cavities is likely due to its 

 internal structure, which originated, in the first place, at the time of the 

 formation of the limestones or dolomites themselves. The eroded beds 

 are not generally those which are largely made up of organic remains, 

 but oftener those which have been due to chemical precipitation. There 

 are, however, exceptions to this general rule which add to the difficulty 

 of accounting for this singular phenomenon. In the process of consoli- 

 dation of chemically formed beds in which lime, magnesia and argilla- 

 ceous matters were present, there would naturally be more or less tend- 

 ency to concretionary action around certain points or centers, thus giving 

 rise to slight differences in composition. Even if such concretionary 

 structure were too obscure to be readily noticed, the probability of its 

 existence in the rocks of the above composition will be readily admitted; 

 but in some cases, especially where oxide of iron is present, this structure 

 may be detected in the form of obscure concentric lines. The globular 

 shape of the cavities is a fact which points to this origin. But, if this be 

 true, it may be asked, why are similar limestones not always eroded in 

 this manner when they are covered by fresh water? The explanation 

 of the difference is probably to be found in a sufficiently acid condition 

 of the water of this part of the lake to slowly dissolve the limestones. 



