1G4 The Ottawa Naturalist. 



world's history, and is seen at the present day in nature in the Great 

 Salt Lake of Utah, as well as at all points where salt is 

 produced by solar evaporation or action. Gypsum is formed principally 

 as a chemical precipitate from solution in water, as well as by the action 

 of sulphurous vapours from volcanic vents upon calcareous rocks. Shell 

 marls are mostly of organic origin, formed by the accumulation of the 

 remains of shells in the bottoms of lakes or ponds, often seen underly- 

 ing peat bogs, as is also the case with certain of the limestones where 

 the mass of the rock is made up almost entirely of organisms. Certain 

 of the limestones, however, are formed by chemical action, by deposition 

 of calcareous matter in solution, in which case they are frequently 

 highly siliceous and devoid of all trace of organic life. Chalk is formed 

 like shell marl, only differing in its being of marine instead of fre^h 

 water origin ; the mass of the deposit being principally calcareous, 

 while with infusorial earth which is formed from portions of diatoms, 

 the mass is chiefly siliceous. This substance although requiring a high 

 power of the microscope to detect the traces of the organisms is often 

 found in deposits of many feet in thickness. 



The deposits of iron ore, which form a very important portion of 

 the economic products of the earth's crust, owe their origin very largely 

 to the action of certain organic acids, which have been produced by the 

 decomposition of vegetable matter upon the ferruginous minerals found 

 in many rock masses, and which thus pass into solution with water. 

 These solutions rapidly decompose under certain conditions and the 

 iron salts are precipitated, and become mixed with sands and clays, 

 gradually forming beds of what is known as bog iron ore. This material 

 in certain areas constitutes deposits of very great extent as in the St. 

 Maurice district, where these ores have been mined and smelted for 

 over 150 years, and are still as abundant as ever, at many points. The 

 other ores of iron, such as limonite, hematite, magnetite &c., which 

 frequently occur in immense masses have also been regarded by some 

 chemists, and geologists as owing their existence to organic agencies, 

 and their present condition is supposed to be due to the great metamor- 

 phoses to which they have been subjected during the great lapse of 

 time since their deposition. It seems however probable from the 





