1882.] OOO fGenth. 



magnetic. It appears that this corundum has not been altered to a very- 

 great extent, only a few specimens of black spinel in irregular masses or 

 rounded, pyramidal forms have been found, besides these only very thin 

 yellowisli or greenish, soft coatings, in very minute quantity, which may 

 be a potassium mica. I could not get enough for examination. The 

 spinel has an iron-black color, and is slightly magnetic. Its specific 

 gravity is = 4.056. 



Mr. George M. Lawrence has made an analysis of it in the Laboratory 

 of the University of Pennsylvania, and found, after deducting 1.47% of 

 silicic acid as follows (a) ; the calculated composition (b). 



100.10 



The titanic acid is present evidently as a mechanical admixture of men- 

 accanite FeTiO^ ; .deducting this and 24.16% of corundum, the composi- 

 tion of the pure spinel is given under (b). I do not consider the FeFe^ a 

 mechanical admixture of magnetite, as it cannot be dissolved out by hy- 

 drochloric acid. 



5. Corundum, altered into Zoisite. 



This is one of the rarer forms into which corundum is altered. I will 

 add, therefore. Towns county, Georgia, as a new locality, where it occurs 

 in small quantity. 



The corundum is of a beautiful pink color, surrounded by greenish- 

 white, cleavable zoisite. 



3. Gorimdum, altered into Feldspar and Mica (Damourite). 



AVhen my first observations on the alterations of corundum were pub- 

 lished, I expressed some doubt about the feldspar, as having been the 

 results of such a change, because I had then not seen any specimens which 

 gave positive evidence of it, although even at that time there was a great 

 probability that a substance which, beyond any question, was found to be 

 altered into fibrolite, cyanite, mica, zoisite, &c., could also, without diffl- 

 cultj^ be converted into feldspars. Since then I have seen many speci- 

 mens which remove my last doubts and prove that most of the occur- 

 rences, referred to in my paper, are the results of alteration. In addition 

 to those already mentioned, I will give a few data which may be of 

 interest : 



a. I had mentioned a granular, yellowish or brownish-white oligoclase 

 from Unionville, as the probable result of such a cliauge. At the same 



