SHRUBSOLE : EROSION OF CERTAIN FRESHWATER SHELLS. 69 



into contact with lime salts, that there is scarcely a water that is 

 wholly free from this soluble salt of lime. The amount present 

 in water may vary from two to ten grains in the gallon, while a 

 spring like that at Holywell contains 24 grains to the gallon. 

 Moreover the quantity of lime present in water is in this way 

 constantly increasing. A spring of water in a limestone country 

 may at its source contain only two grains, and in its subsequent 

 progress over limestone rocks may increase the amount to twenty 

 grains in the gallon. The explanation is that in passing along 

 it has dissolved out a portion of the limestone. The idea here 

 present to my mind was that under certain circumstances (such 

 as the presence of carbonic acid), water might in a similar way 

 act upon the shelly matter of the mollusca, in the absence of 

 other material, and so give rise to the erosion seen upon the 

 shells. 



Before leaving this, the theoretical side of the question, I 

 want it to be clearly understood that the substance of which 

 shelly matter is composed is one that would be acted upon by 

 all waters, since carbonic acid is invariably present. The next 

 point is that the power of so acting upon the shell is in inverse 

 ratio to the quantity of calcic carbonate already present in the 

 water. Thus water containing only two grains of it in the 

 gallon would have a more powerful action upon shelly matter 

 than if it contained ten grains, and still greater action than if it 

 held twenty grains in the same amount of water. There is a 

 limit to the dissolving power of water, and in time a saturated 

 point is reached, the water will take up no more, and all action 

 upon the shell will cease. 



We now take our leave of theories and come to the facts 

 brought out by the revelations of the several analyses. The 

 first thing is the amount of the lime salt present in the water ; 

 in this we see the work done by the carbonic acid in presenting 

 the lime in a soluble form. In the Trent Canal there were 

 present 8"33 grains of lime in solution, as against an average of 

 3 grains only in the Dee water, — a marked difference. It would 



