and the allied Rocks. 61 



was observed by Dr. Jackson at Keweena Point, Lake Su- 

 perior. The datholite is often found there in veins with na- 

 tive copper, and is associated in some places with a curious 

 slag of boro-silicate of iron and copper. Sometimes the cry- 

 stals of datholite, as well as the prehnite and calc-spar, con- 

 tain scales or filaments of native copper. These very im- 

 portant observations seem to establish the same origin for the 

 three minerals, for Dr. Jackson states that they appear to be 

 contemporaneous ; and if calc-spar has been deposited from 

 a solution, the same holds true of the others. They have all 

 been formed subsequent to the copper filaments of the cavities, 

 for they were deposited around them ; yet may have been the 

 next to form during the cooling of the rock. The boro-sili- 

 cate of iron and copper has resulted from the same causes. 



Analcime approaches the zeolites in composition, but like 

 the prehnite and datholite it contains less water, and is very 

 different in its crystallization. We have less evidence as to 

 the heat necessary for its formation ; yet it was probably 

 formed at a somewhat elevated temperature. 



With regard to the other amygdaloidal minerals, we are in 

 still greater doubt as to the necessity of heat. We cannot at 

 present fully appreciate the efficiency of chemical agents in a 

 nascent state acting slowly without heat through long periods. 

 Many of them may require heat, and some may be the last 

 depositions from the filtrating waters after they have nearly 

 or quite attained their reduced temperature. But the forma- 

 tion of zeolitic stalactites in caverns favours the view that some 

 at least may form at the ordinary temperature by the slow 

 decomposition of the containing rock after it had emerged 

 from the waves # . Kersten has lately described a modern 

 stellated zeolite forming incrustations on the pump-wells of 

 the Himmelsfahrt mine near Freyberg. It consisted of silica, 

 oxides of iron and manganese and water. Further examina- 

 tion will probably bring more of these modern products to 



The formation of particular minerals in certain regions de- 

 pends of course upon the supply of the necessary ingredients. 

 Where the supply of lime has been large, we should expect 

 to find some of the minerals, prehnite, Heulandite, Laumonite, 

 stilbite, scolecite, dysclasite, chabazite, for carbonate of lime 

 decomposes the silicates of potash and soda. Instances of 



* Annates des Mines, ii. (4th Ser.) 465, 1842. 



•f* Carbonate of iron seems never to form from water at the surface, its 

 solutions depositing a hydrated peroxide of iron instead of the carbonate; 

 it may therefore require a submerged condition of the rock, although not 

 necessarily a raised temperature. 



