294 On the Composition of Corals. 



adds additional probability to these conclusions ; for although fluorine 

 has not yet been detected in these polyp remains by analysis, fluor- 

 spar is a common mineral in fossilit'erous limestones, and often occurs 

 in the cavities of shells and other fossils, as if proceeding from animal 

 decomposition. 



The same heat, then, that crystallized the limestone, crystallized 

 also the apatite, splendid localities of which occur in the limestone of 

 Northern New York, as well as in Orange county. It is universally 

 admitted that this crystallization took place under the pressure of an 

 ocean, and we may believe that the heat was di.-tributed by means of 

 its waters, both permeating and superincumbent. 



These heated waters, like the hot waters of igneous regions gene- 

 rally — fine examples of which are seen in New Zealand, as well as 

 the Geysers of Iceland — contained silica in solution. Through the 

 action of this silica on the lime and magnesia, and on the oceanic 

 salts present, may have been formed the minerals serpentine., scapo- 

 lite, piyroxene, tremolite, &c., so common in granular limestone and 

 dolomite. Another source of silica is found in the clay which sedi- 

 mentary limestones often contain, hydraulic varieties of which some- 

 times include 40 per cent. A hydraulic limestone from the Hel- 

 derberg. New York, analyzed by Professor Beck, contained 36 per 

 cent., 28 per cent, of which were silica ; and another from Rondout, 

 Ulster county. New York, contained 24-50 per cent, of clay, 15'37 

 of which were silica, and 9' 13 alumina. This may account for the 

 occurrence of the above minerals without the infiltration of silica ; yet 

 the large amount of silica or siliceous minerals in some granular lime- 

 stones, seems to require moi'e than can be reasonably supposed to be 

 supplied from this source. Bloreover, the occurrence of siiica in solu- 

 tion in regions of volcanic agency, is known to be frequent, and 

 cannot in all instances be excluded for accounting for these metamor- 

 phic changes and crystallizations. 



The alumina of the same clay in impure limestones uniting with 

 the magnesia and silica, may have given rise to the aluininous va- 

 rieties of hornblende; and the iron often present, may have contributed 

 towards producing the dark ferruginous varieties. The formation 

 of mica may be explained on the same principles. 



The presence of fluorine has already been alluded to ; and this, 

 with the silica and magnesia, appears to have produced the ehondro- 

 dite, another very abundant mineral in our dolomites. 



The alumina and silica with potash or soda from heated volcanic 

 waters might have originated crystals of feldspar, albite, &c. 



Where spinels have been formed, the alumina of the altered mag- 

 nesian limestone may have predominated over the silica, and thus 

 have given rise to this mineral — an aluminate of mngnesia. Spinel 

 is usually associated in Orange county, New York, with chondrodite. 

 And it seems probable that the little fluorine present, at once took 

 posse'-sion of the silica, and formed fluosilicic acid, — for the attraction 

 between fluorine and silica is known to be one of the strongest in 



