Mr. Dana on the Composition of Corals , fyc. 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. Dapm * 



ab! 



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- 

 vestigations 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- 

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

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

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

 e «ce in their composition. The phosphates constituted in some instan- 

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



. hese fects seem to prove, what has long been suspected, that the 

 primary limestones and dolomites are altered sedimentary limestones, 

 an 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- 

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

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

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

 n d ^ te cted in these polyp remains, by analysis, fluor spar is a com- 

 ° n mine ral in fossiliferous limestones, and often occurs in the cavities 

 ,1 Is an( * otller fossils, as if proceeding from animal decomposition. 

 he **m heat, then, that crystallized the limestone, crystallized also 



e a Patite, splendid localities of which occur in the limestone of north- 

 ed h ew York as weU as in 0ran 8 e Count y- h is universally admit " 



* foat this crystallization took place under the pressure of an ocean, 

 ^ *e may believe that the heat was distributed by means of its waters, 

 Permeating and superincumbent. 



e se heated waters, like the hot waters of igneous regions gene- 



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



e Geysers of Iceland— contained silica in solution. Through the ac- 



0n of this silica on the lime and magnesia, and on the oceanic salts 



r eSent > rna y have been formed the minerals serpentine, scapolite, py- 



X6ne > trctnolite, &c. so common in granular limestone and dolomite. 



Th 



iti«„ , forc tho Association of American Geologists and Naturalists at Wasli- 



» t0n » May, 1844. 





