136 Mr. Dana on the Composition of Corals^ SfC. 



Another source of silica is found in the clay which sedimentary lime- 

 stones often contain, hydraulic varieties of which sometimes include 40 

 per cent. A hydraulic limestone from the Helderberg, N. Y., analy- 

 zed by Prof. Beck, contained 36 per cent., 28 per cent, of which were 

 silica ; and another from Rondout, Ulster County, N. Y., 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 limestones, seems to require more than can 

 be reasonably supposed to be supplied from this source. Moreover, 

 the occurrence of silica in solution in regions of volcanic agency, is 

 known to be frequent, and cannot in all instances be excluded in ac- 

 counting for these metamorphic changes and crystallizations. 



The alumina of the same clay in impure limestones uniting with the 

 magnesia and silica, may have given rise to the aluminous varieties 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 chondrodite, an- 

 other very abundant mineral in our dolomites. 



The alumina and silica with potash or soda from heated volcanic wa- 

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



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

 sian limestone may have predominated over the silica and thus have 

 given rise to this mineral — an aluminate of magnesia. Spinel is usu- 

 ally associated in Orange County, N. Y., with chondrodite. And it 

 seems probable that the little fluorine present at once took possession of 

 the silica, and formed fluosilicic acid, — for the attraction between fluo- 

 rine and silica is known to be one of the strongest in chetnistry--and 

 thus combined, produced by uniting with magnesia the mineral chon- 

 drodite, which is a compound of silica, fluorine, and magnesia. Where 

 spinels are not found, as with rare exceptions is the case in St. Law- 

 rence County, N. Y., it may be owing to the large amount of silica 

 present, or the absence of fluorine. Chondrodite is nearly as rare a 

 mineral in northern New York as spinel 



