On t/ie Composition of Corals. 293 



early writings, I liave spoken of under the title of the Volcano 

 Storjn. The hot watery vapour which rises from the crater, 

 and mingles with the atmosphere during the eruption, forms a 

 cloud as it cools, with which the column of ashes and fire, 

 many thousand feet in height, is surrounded. So sudden a 

 condensation of vapour, and the production of a cloud of enor- 

 mous superficial dimensions, increase the electrical tension, as 

 Gay Lussac has shewn. Forked lightnings dart from the 

 column of ashes, and the rolling thunder of the volcanic storm 

 is then plainly distinguished from the rumbling in the interior 

 of the mountain. This was well observed towards the end of 

 the eruption of Vesuvius, in the month of October 1822. The 

 lightning, which proceeded from the volcanic steam- cloud of 

 the Katlagia burning mountain in the island of Iceland, accor- 

 ding to Olafsen's account, upon one occasion (17th October 

 1755) killed eleven horses and two men. 



On the Composition of Corals and the Production of the Phos- 

 phates, Aluminates, Silicates, and other Minerals, by the Me- 

 tamorphic 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 junior, who had been engaged 

 in chemical examinations of the corals of the Exploring Expedition. 

 These investigations have been continued since, and lead to the con- 

 clusion that this is, beyond doubt, the source of the magnesia in macr- 

 nesian-limestone. The analyses are not yet completed, and I cannot 

 therefore give definite results. 



Besides finding magnesia, Mr Silliman has made the more re- 

 markable discovery of a large per-centage of phosphoric acid. Ana- 

 logy had taught us that corals correspond, in their nature and rela- 

 tions, to the bones of higher animals; and we have now further evi- 

 dence of this correspondence in their composition. The phosphates 

 constituted, in some instances, 9 or 10 per cent, of the whole. 



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

 primary limestones and dolomites are altered sedimentary limestones, 

 and that these limestones may bo in part of coral origin. The so 

 frequent occurrence of phosphate of lime (apatite) in this rock, is ex- 

 plained by the same discoveries, and corroborates this view of its 

 origin. The little fluorine which apatite contains (6 or 8 percent ), 



* Read before the Association of American Geologists and Naturalists, 

 at Wasliin^ton, May 1644. 



