136 



replacement of the constituents of certain minerals, the crystalline forms 

 of which remain the same. See Memoirs Bost. Nat. Hist. Soc.,ii., p. 88. 



GIBBSITE. 

 (Mineralogy of New- York, page 320.) » 



Hermann, of St. Petersburgh, has announced that the constitution of 

 gibbsite was that of a hydrous phosphate of alumina, and that the com- 

 position assigned to gibbsite by Torrey, belonged only to the hydrargil- 

 lite of Rose. Prof. B. Silliman, Jr., has repeated the analysis of this 

 mineral, from Richmond, Mass , and finds it to correspond to the formula 

 first given, which requires : 



Alumina, 65-800 



Water, 34-200 



100-000 

 And the mean results of his analysis correspond very closely to the 

 calculated per centages. The phosphoric acid is like the magnesia, iron 

 and silica, contained in gibbsite, only as a contingent impurity. The 

 gibbsite, he says, is sometimes mixed with allophane, which will account 

 for the presence of silica ; and he thinks the silica mentioned by Dr. 

 T. Thompson, in his analysis, was derived from the same intermixture 

 of the two species. There is now little doubt that the hydrargillite of 

 Rose, and gibbsite, are identical ; the former is the crystalline, the lat- 

 ter the amorphous variety of the same species. Sill. Jour., July, 1849 

 page 411. 



IDOCRASE. 



(Mineralogy of New- York, page 321.) 



Subsequent examination has satisfied me that the mineral called ido- 

 crase, from Hall's, on Muscolunge lake, in Jefferson county, is pyroxene. 

 The measurement of the primary is, as nearly as can be determined, 

 87° and 93° ; the replacing planes, 134°. Its color, although peculiar, 

 is not unlike that of the pyroxene found on the Rossie turnpike, two 

 miles from the village of Oxbow, in Jefferson county. 



GARNET. 

 (Mineralogy of New- York, page 323.) 



The variety colophonite, is reported by Dr. Emmons as being abun- 

 dant at Johnsburg, Warren county. Rep. on the Gcol. 2d Dist., p. 192. 



