Mr J. D. Dana on the Ori(jin of Trap Minerah. 2G0 



ceous waters contained ; and probably the lime as well as silica 

 was derived in part from an external source. The pseudo- 

 morphs prove that Prehnite may have been the result also of 

 subsequent eruptions, at the same time that they shew 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 per-cen- 

 tage of water, 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 evolved about volcanoes or in vol- 

 canic regions. The hot lagoons of Tuscany, and the volcano 

 of Lipari, are the most noted examples. 



Although ]>oracic 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 products. 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 ma- 

 rine salts ; and all are so distributed over the arid country, 

 that the region has been lately described as having been be- 

 yond 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.* 



If this be its origin, the necessity of heat and pressure 

 must be admitted, in order to produce the chemical combina- 

 tions in datholite. Its elements are not those of the felspar 

 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 



* The only other known source is the mineral tourmaline, quite an improba- 

 ble 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. 



