BIr. Dana on the Composition of Corals, S^c. 135 



Art. IX. — On the composition of Corals and the production of the 

 phosphates^ aluminates, silicates, and other minerals, by the metamor- 

 phic action of hot water ; by J. D. Dana.* 



At the last meeting of this Association I alluded to the discovery of 

 magnesia, in corals, by B. Silliman, Jr., who had been engaged in chem- 

 ical examinations of the corals of the Exploring Expedition. These in- 

 vesti'^ations have been continued since, and lead to the conclusion that 

 this is beyond doubt, the source of the magnesia in magnesian limestone. 

 The analyses are not yet completed and I cannot therefore give defi- 

 nite results. 



Besides finding magnesia, Mr. Silliman has made the more remarka- 

 able discovery of a large per centage of phosphoric acid. Analogy had 

 taught us that corals correspond in their nature and relations to the bones 

 of higher animals, and we have now farther evidence of this correspon- 

 dence in their composition. The phosphates constituted in some instan- 

 ces, 9 or 10 per cent, of the whole. 



These facts seem to prove, what has long been suspected, that the 

 primaiy limestones and dolomites are altered sedimentary limestones, 

 and that these limestones may be in part, of coral origin. The so fre- 

 quent occurrence of phosphate of lime (apatite) in this rock, is explain- 

 ed by the same discoveries, and corroborates this view of its origin. 

 The little fluorine which apatite contains, (6 or 8 per cent.) adds addi- 

 tional probability to these conclusions ; for although fluorine has not yet 

 been detected in these polyp remains, by analysis, fluor spar is a com- 

 mon mineral in fossiliferous 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, ciystallized also 

 the apatite, splendid localities of which occur in the limestone of north- 

 ern New York as well as in Orange County. It is universally admit- 

 ted that this crystallization took place under the pressure of an ocean, 

 and we may believe that the heat was distributed 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 ac- 

 tion of this silica on the lime and magnesia, and on the oceanic salts 

 present, may have been formed the minerals serpentine, scapolite, py- 

 roxene, tremolite, &c. so common in granular limestone and dolomite. 



* Read before the Association of American Geologists and Naturalists at Wash- 

 ington, May, 1844. 



