POSSIBLE ORIGINS OF THE EROSION. 301 



which has been rendered classic by Lyell in his " Principles of Geol- 

 ogy," will occur to every geologist. The cavities formed by boring 

 mollusks are, however, burrows which are of pretty uniform calibre, or 

 increase in diameter with the depth, owing to the growth of the animal as 

 it i)roceeds. They are generally many times deeper than wide, whereas 

 those under consideration are either glol)ular or short by comparison, 

 and they contract instead of enlarge toward the bottom. 



ACTION OF SPONGES AND ALG.E. 



A small sponge, Cliona cclata, makes burrows in the shells of oysters, 

 and this fact led to the supposition that possibly some species of fresh- 

 water sponge might have aided in deepening or enlarging the cavities 

 under discussion, or in determining their form by the production of 

 small quantities of an organic acid, either during life or upon decompo- 

 sition, but no evidence could be found in support of this idea. The re- 

 mains of branching fresh-water algte may be seen in some of the cavities, 

 but they do not appear to have exercised the least influence in their 

 production. The lower or jelly-like alga? of fresh water do, however, 

 l)ossess the power of dissolving limestone. The journal of the Royal 

 Microscopical Society for October, 1894, says, on page 597, that — 



** Professor F. Cohn points out the important part played by very lowly organ- 

 ized algic — Phycochromaceae and Cyanophyee?e — in the formation of calcareous 

 and silicious rocks. Many beds of marble and travertine have been formed in 

 this way. He further enumerates the algse that are known to destroy calcareous 

 rocks by erosion. In all fixed algve there appears to be this contrast between the 

 basal cells and the rest of the filament; that the former excrete an acid which 

 dissolves lime, wliile the latter has the power of depositing a soluble lime-salt be- 

 tween the filaments, but within the mucilage which is excreted from the sheath." 



WEARING ACTION OF PEBBLES. 



A popular notion current among many of the residents of the localities 

 where the pitted surfaces occur is that the cavities have been worn by 

 tlie whirling of pebbles and sand in a manner analogous to the forma- 

 tion of potholes at ra[)ids and falls. That they have not been thus formed 

 is obvious from the following reasons, selected from among others : 



Their shaj)es do not correspond with this mode of formation. 



They occupy the whole surface of the rock, whereas potholes occur at 

 irregular intervals. 



The walls of the cavities are generally uneven or rough, and deiicate 

 silicified fossils often project from them or stretch completely across the 

 cavities, whereas the wearing action of rotating pebbles and sand would 

 have produced smooth and cylindrical walls. 



No pebbles are ever found in them except such as can be shown to 

 have been placed there subsecjuent to their excavation. W these cavities 



