300 R. BKLL: — HONEYCOMBED LIMESTONES IN LAKE HURON. 



otherwise it would be of a more uniform character, since it is due to some 

 common external cause operating alike upon them all. 



The dissolving of the unaltered limestones or dolomites goes on at 

 right angles.to the bedding or directly downward or upward, as the rocks 

 are practically horizontal, never at an oblique angle nor horizontally, 

 which, in the absence of some inhibiting cause, might easily take place 

 in loose masses which lie at all angles on the bottom of the lake. On 

 isolated blocks, where the sides have been as freely exposed to the water 

 as the top, the solvent process appears to prefer to work downward from 

 the " quarry bed " and not to eat inward from the sides. 



Experiments made by Professor Goodwin, of Queen's University, 

 Kingston, on the action of solvents on slowly soluble substances seem to 

 show that the tendency of solution is strongest directly upward and 

 downward. This, together with a faint concretionary structure, to be 

 noticed further on, may help to account for these forms of erosion. 



Allusion has been made to a well marked case, where the pitting has 

 progressed directly upward from the under surfaces of beds of pure lime- 

 stone forming the roof of an overhanging ledge or shelf which had been 

 submerged when the lake was a little higher than it is at present. This 

 occurs along the east side of The Narrows between Little Cloche island 

 and the southern part of Cloche peninsula. The rock, which belongs to 

 the Black River formation, consists of a soft bluish grey limestone con- 

 taining a little argillaceous matter. The tapering finger-shaped pits are 

 closely crowded together and they penetrate upwards from two to four 

 inches from the general outline of the roof. Silicified fossils project' 

 from the walls into these pits, and when the rock is broken across a dis- 

 coloration of iron oxide is seen to extend a short distance all round the 

 wall of each one. Plate 14 illustrates these pits. 



Possible Origins of the Erosion. 

 borings of mollusks. 



The writer may not have arrived at a correct explanation of these 

 curious forms of erosion, but he will endeavor to state the suggestions 

 which have occurred to him in regard to their origin, along with a de- 

 scription of the circumstances connected therewith. Many theories to 

 account for these phenomena may present themselves both to those who 

 have and those who have not seen them on the ground, and it may be 

 as well here to notice briefly the more obvious ones. 



Cavities resembling some of those above described are made in rocks 

 by boring mollusks, such as Saxicava, Pholas, Petricola, QiUista, Tapes, 

 Venerupis and Lithodomus.. The case of the zones on the marble pillars 

 of the temple of Jupiter Serapis, at Puzzuoli, eroded by lithodomi, 



