270 Mr J. D. Dana on the Origin of Trap Minerals. 



submarine eruption, and permeated the bed of molten rock 

 shortly after ejection. Thus placed in circumstances of pres- 

 sure and confinement, along with silica in solution, the vola- 

 tile boracic acid might enter into the combination presented 

 in datholite. 



An interesting fact bearing upon the history of datholite, 

 was observed by Dr Jackson at Keweena Point, Lake Supe- 

 rior. The datholite is often formed there in veins with native 

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

 of boro-silicate of iron and copper. Sometimes the crystals 

 of datholite, as well as the Prehnite and calc-spar, contain 

 scales or filaments of native copper. These very important 

 observations seem to establish the same origin for the three 

 minerals — for Dr Jackson states that they appear to be co- 

 temporaneous ; 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 cavi- 

 ties, for they were deposited around them ; yet may have been 

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

 silicate 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 efiiciency 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 de- 

 positions from the filtering waters, after they have nearly or 

 quite attained their reduced temperature. But the formation 

 of zeolitic stalactites in caverns favours the view that some at 

 least may form at the ordinary temperature, by the slow de- 

 composition 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 Him- 



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



