On the Minerals of Trap and tie allied Rocks. 61 
nated at a somewhat more elevated temperature than the other 
species—the same conclusion that is drawn from their lower po- 
sition in geodes. 
The fact, also, that Prehnite has been found forming pseudo- 
morphs, bears the same way ; for heat would be necessary, in all 
probability, to aid in removing the original mineral. 'The vast 
extent of some Prehnite veins—occasionally, as Dr. Jackson has 
observed, three or four feet wide—refers to an origin like that of 
the quartz in similar rocks. Indeed, there seems little doubt that 
Prehnite is often derived from that portion of the silica in solution 
which entered into combinations at the time with the alumina 
and lime which the siliceous waters contained ; and probably the 
lime as well as silica was derived in part from an external source. 
The pseudomorphs prove that Prehnite may have been the result 
also of subsequent eruptions, at the same time that they show 
the probable necessity of heat for its formation. 
Datholite is a compound of silica, lime and boracic acid, with - 
about 5 per cent. of water. Besides the small percentage of wa- 
ter, and its being, next to quartz, the lowermost mineral in geodes, 
we find an additional fact, alone almost decisive with regard to its 
origin, in its containing boracic acid. Boracic acid is often evolv- 
ed about volcanoes or in volcanic regions. The hot lagoons of 
Tuscany, and the volcano of Lipari are the most noted examples. 
Although boracic acid has never been detected in sea water, 
there can be little doubt of its occurring in it. The usual modes 
of analysis by evaporation would dissipate it, and of course it could 
not thus be detected except with special care and by operating 
on a large quantity of water. Borate of soda (boracite) is 
found only in beds of salt and gypsum,—both sea-water pro- 
ducts. Moreover, borate of lime has been lately found on the 
dry plains in the northern part of Chili, along with common salt, 
iodic salts, gypsum and other marine salts, and all are so distri- 
buted over the arid country, that the region has been lately de- 
scribed as having been beyond doubt once the bed of the sea. 
These facts render it altogether probable that sea water which 
gains access to volcanic fires is the source of the boracic acid in 
volcanic regions.* 
* The only other known source is the mineral tourmaline, quite an improbable one 
in the case before us. It is possible that tourmaline may have received its boracic 
acid from the sea during granitic eruptions, and the occurrence of this mineral in 
the vicinity of trap dykes is explained in the same manner. 
