60 J. D. Dana on the Minerals of Trap 



portion of the silica in solution which entered into combina- 

 tions at the time with the alumina and lime which the silice- 

 ous 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 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 percent- 

 age 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 boracic acid has never been detected in sea-water, 

 there can be little doubt of its occurring in it. The u.^ual 

 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 marine 

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

 region has been lately described 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*. 



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

 be admitted, in order to produce the chemical combinations 

 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 sub- 

 marine 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 



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

 probable 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. 



