

Intelligence and Miscellaneous Articles. 245 



ON THE OCCURRENCE OF FLUOR SPAR, APATITE AND CHON- 

 DRODITE IN LIMESTONE. BY JAMES D. DANA. 



The analyses of corals by Mr. B. Silliman, jun.*, have shown that 

 although ordinary corals consist mainly of carbonate of lime, there is 

 present a small proportion of phosphates and fluorides, with some 

 silica, alumina and oxide of iron. It is also probable, from some 

 trials by Mr. Silliman, that these constituents exist also in many 

 shells. 



From the results obtained in these analyses, it appears that the 

 fluorides and phosphates amount, on an average, to about ^ per cent., 

 or 0'25 part in a hundred parts of coral ; and the amount in the 

 same manner of the phosphates, is - 05 per cent. A cubic foot of 

 coral, as deduced from the average specific gravity ascertained by 

 Mr. Silliman, weighs 157 pounds. Consequently in each cubic foot 

 there are Q\ ounces of fluorides and 1£ ounce of phosphates ; in 

 each cubic rod, 1700 pounds of fluorides and 340 pounds of phos- 

 phates. These fluorides are fluorides of calcium and magnesium, 

 and the phosphates are phosphates of lime and magnesia. In the 

 same manner we ascertain that the amount of fluorides in a reef of 

 coral, a mile long, half a mile wide and a hundred feet deep, amounts 

 to more than 500,000,000 pounds. The proportion of silica is a 

 little less than that of the fluorides. 



Late geological researches have placed it beyond doubt, that the 

 various limestones consist mainly, like coral limestone, of animal 

 remains, among which corals in many instances hold a conspicuous 

 place. These limestones often contain crystallizations of fluoride of 

 calcium (fluor spar) ; and in other beds which have evidently been 

 acted upon and crystallized by heat, there are also apatite (phosphate 

 of lime) and chondrodite (composed of fluoric acid, magnesia and 

 silica). Moreover these are the most common minerals of these 

 limestones. 



The above deductions supply us with a full explanation of the 

 origin of these minerals. The fluorine, phosphoric acid, lime, mag- 

 nesia and silica present, are adequate for all the results, without 

 looking to any other sources. Instead therefore of being extraneous 

 minerals introduced into the limestone rock, their elements at least 

 are an essential part of its constitution ; and they have been sepa- 

 rated from the general mass by a segregation of like atoms under 

 well-known principles, and it may be arranged anew, in some cases, 

 according to their affinities. Fluoride of calcium may crystallize 

 out when under water without much or any heat ; and it is an in- 

 teresting fact, that this fluoride has been lately proved by Mr. G. 

 Wilson, to be soluble to some degree in pure cold waterf. Mr. 

 G. Wilson has also shown that fluorides actually exist in sea- water, 

 as had been suggested by Mr. Silliman some months before the dis- 



* See the volume of the Exploring Expedition on Zoophytes, p. 7\% ; 

 and Silliman's American Journal, Second Series, vol. i. p. 189. 

 t See Chem. Gazette, No. 85, May 1846, p. 183. 



