124 



MINERALS AND GEOLOGY 



Crystallized examples occur likewise in hollows and in fissures of 

 many of our Silurian and Devonian strata, as more especially, in the 

 Trenton limestone near Lachine, and in the same formation in the 

 township of Huntingdon in Hastings County ; in dolomitic beds of 

 the Quebec group near Point Levis, opposite Quebec ; and in the 

 Niagara formation in the vicinity of the Great Falls, Hamilton, 

 Dundas and elsewhere. Many of the amygdaloidal trap rocks of 

 Lake Superior and Lake Huron, also, enclose nodular cleavable 

 masses of calcite, and occasionally the more open amygdaloidal 

 cavities are lined with crystals. These are almost always scalenohe- 

 drons, or combinations in which one or more scalenohedrons pre- 

 dominate. 



(b) Concretionary and Stalactitic varieties of Calcite : These 

 varieties are being constantly formed by deposition of carbonate of 

 lime from springs and streams in limestone districts, and from water 

 percolating through limestone rocks. Carbonate of lime, consisting 

 of equal atoms or combining weights of lime and carbonic acid, is 

 comparatively insoluble in water, but the bicarbonate, containing 

 two parts of carbonic acid to one of lime, dissolves to a certain ex- 

 tent. Water contains very generally a small amount of free car- 

 carbonic acid, derived from the atmosphere, decaying organic matter, 

 &c., and thus it is enabled to take up a certain quantity of carbonate 

 of lime, this becoming converted into bicarbonate. The latter com- 

 pound, however, is extremely unstable. It parts with carbonic acid 

 very readily, even by simple exposure to the air. The insoluble 

 carbonate thus again results, and is necessarily precipitated from the 

 water, the precipitation often taking place upon moss, roots and 

 other organic bodies, converting these into so-called " petrifactions." 

 Water issuing from limestone strata 

 often deposits concretionary masses 

 of carbonate of lime, in this manner, 

 as at Hamilton, Rockwood, the 

 Falls of Noisy River, the Banks of 

 Beaver River in Euphrasia and 

 Artemisia, and other places along FIG. 76. 



the escarpment of the Niagara Formation (see Part V.). Deposits 

 of this kind are commonly known as Calcareous Tufa. Specimens 

 from Hamilton, more especially, are hard and solid, and admit of a 



