298 R. BELL — HONEYCOMBED LIMESTONES IN LAKE HURON. 



Grand Manitoulin island is 80 miles in length. Along its southern 

 side the undisturbed Silurian limestones slope very gently southward 

 under the lake, so that shallow water extends a considerable distance 

 from shore. On surfaces which have been long exposed to the weather 

 or to the wearing action of the waves the pitting is more or less effaced, 

 but wherever they have been covered by the water or otherwise sheltered 

 it is still plainly visible. In the autumn the whitefish come into shallow 

 water to spawn, and the fishermen say that the rough limestone bottom, 

 here described, is a favorite resort for this purpose. As its inequalities 

 protect the eggs from destructive currents and predaceous fishes, it may 

 be said to serve an economic purpose. 



Physical Characteristics of the eroded Rocks. 



Various kinds of limestones have been acted upon by this peculiar 

 form of erosion. The unaltered varieties have been completely riddled 

 with cavities, varying from very small holes up to three inches or more 

 in diameter, the average being between half an inch and an inch, but in 

 altered or crystalline limestones or dolomites the pits are mostly larger and 

 shallower. The pitting of these rocks will be more fully noticed further 

 on. In unaltered dolomites one form assumed by the cavities is globular 

 to pear-shaped, and in the process of enlargement they encroach upon 

 one another until only thin walls remain between them, while others 

 coalesce, and ultimately the whole mass becomes separated into a highly 

 eroded skeleton, as shown in figures 1 and 2, plate 13, taken from photo- 

 graphs. Even in this stage, the solid angles between adjacent cavities 

 become perforated by smaller holes, and at length the rock crumbles to 

 fragments, with deeply hollowed concave surfaces. The removal of such 

 a wasted exterior exposes a pitted surface on the next lower layer of lime- 

 stone, the contiguous cup-shaped hollows usually occupying the whole 

 area. A completely sculptured surface resembling this may also be pro- 

 duced by the direct solvent action of the water without the intervention 

 of the globular honeycombing. Large areas showing surfaces of this 

 kind, which had been eroded at a considerable depth, may now be seen 

 in the clear, shallow water or exposed to the air, owing to the lowering of 

 the level of the lake, while wide borders of nearly horizontal limestone 

 beds, similarly eroded, are exposed in some localities between the mar- 

 gin of the water and the wooded shore. These extremely eroded sur- 

 faces have a striking appearance, and the sharp and pointed edges of the 

 pits would be painful to walk on without thick soled boots. 



Another form which these cavities take is finger-shaped, crowded to- 

 gether and deeply indented, or of such a shape as would be completely 

 filled by inserting the sharp end of a cigar. As in the other forms, the 

 cavities at their tops adjoin each other, leaving no space between them. 



