120 ROCK-METAMOEPHISM. 



SUPPLEMENTAKY NOTE C. 



CEKTAIN LIMESTONES ARE OF MECHANICAL ORIGIN. 



IT is well known that Dr. Sterry Hunt, in advocating the 

 purely chemical origin of limestones, places himself in op- 

 position to the opinion generally prevailing on this subject, 

 viz. that while some rocks of the kind are chemical products, 

 such as freshwater and marine travertines, by far the greater 

 portion are of organic origin, that is, the skeletal exuviae of 

 shells, corals, foraminifers, and other animals. Excluding the 

 methylosed crystalline limestones, I quite agree with this view ; 

 but I have now to bring under notice another class of calcareous 

 deposits, whose origin, it is assumable, was altogether different ; 

 though they may have been primarily of organic, chemical, or 

 methylotic development, 



The calcareous nature of the erratic drift, so well developed 

 near the " Citie " of Galway (to which reference has been pre- 

 viously made, footnote, p. 93) , can only be due to this deposit 

 having been derived from the Carboniferous limestone of the 

 surrounding district by the abrading action of glaciers. From 

 what has lately come under my notice I see no reason why 

 certain calcareous deposits, obviously of littoral origin, cannot 

 be the debris of mechanically abraded limestones. Lately, 

 availing myself of an occasion when Lough Corrib was in a 

 muddy condition, caused by heavy rains, I put aside a couple of 

 quarts of the water to stand for a few days* Testing the 

 sediment which had settled at the bottom of the vessel with 

 hydrochloric acid, a brisk effervescence took place, denoting it 

 to contain a notable quantity of carbonate of lime. The 

 remaining clear water, on being tested with oxalate of ammonia, 

 exhibited decided evidence of its containing bicarbonate of lime 

 in solution. These facts prove that the water of rivers in 

 limestone districts contains calcareous matter both mechanically 



