62 On the Minerals of Trap and the allied Rocks. 
If this be its origin, the necessity of heat and pressure must be 
admitted, in order to produce the chemical combinations in datho- 
lite. Its elements are not those of the feldspar or other trap 
minerals, like the zeolites superimposed on it; but they have come 
from an extraneous source, and none is more probable than the 
sea waters, which were heated at the submarine eruption, and 
permeated the bed of molten rock shortly after ejection. ‘Thus 
placed in circumstances of pressure and confinement, along with 
silica in solution, the volatile 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 Superior. 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 cotemporaneous—and if calc spar has been de- 
posited 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- 
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 dif- 
ferent 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 formation of zeolitic stalactites in 
caverns favors the view that some at least may form at the ordi- 
nary temperature by the slow decomposition of the containing 
rock after it had emerged from the waves.* Kersten has lately 
* Annales des Mines, ii, (4th Ser.) 465, 1842. 
