J882.] ^ol [Genlh. 



Contributions from the Laboratory of the University of 

 Pennsylvania. 



No. XX. 



CONTRIBUTIONS TO MINERALOGY. 



By F. a. Genth. 



iRead before tlie American Philosophical Society, August 18, 1SS2.) 



I. In a paper, read before tke American Philosopliical Society on Sep- 

 tember 19th, 1873, I communicated some observations on the occurrence 

 of Corundum, and, especially, on its alteration into other minerals. Since 

 then I had an opportunity to examine many beautiful specimens of the 

 same kind, by which my views on the subject received the fullest confir- 

 mation. I was in hopes that I would be able to prepare a second edition 

 of my paper, illustrated with carefully drawn, figures of the most import- 

 ant and striking forms, but, finding that my time is too much taken up by 

 other duties, I fear that I shall never accomplish my desire, and, for this 

 reason, will place on record, as an appendix to my first paper, the de- 

 scription of a few very remarkable occurrences.* 



* In his Handbuch der Mineralclieraie, 3 Auflage, Leipzig, 1875, Prof. C. F, 

 Rainmelsberg repeatedly refers to the above investigation, but, unfortunately, 

 gives me credit for statements which I never have made. As they are of too 

 much imijortance to remain uncontradicted, I Avill briefly allude to the most 

 striking. 



On page 147 (Specieller Theil), he says that I came to the conclusion that " at 

 the time, when chrysolite changed into serpentine, corundum was formed, which, 

 subsequently, was altered into other minerals," while I simply state the fact that 

 the largest deposits of corundum occur in serpentine, or in chrysolite, or the 

 rocks iiumediately adjoining the same, and I do not even intimate that they 

 were formed at the time when the latter changed into the former, as they 

 occur equally in both ; that I do not svippose (as Prof. R. seems to believe) that 

 the alumina was eliminated from rocks which do not contain any appreciable 

 quantity of it, is, I should think, sufficiently indicated by my query, " by what 

 agencies such enormous quantities of alumina coidd have been precipitated to 

 form corundum ?" 



On page 137, in quoting some ot my analyses of the black spinels (ceylanites), 

 Prof. Rammelsberg remarks that their purity was verydoubtful,a.nd that besides 

 the A. 31 per cent, of corundum, which were eliminated during the process of 

 analysis, it must contain 9.6 per cent, additional. It Prof. Rammelsberg had read 

 my paper with the least attention, lie would have found that 1 come pretty 

 nearly to the same conclusion, tor I say : ^^ that the most carefully selected ma- 

 terial still contained amechanical admixture of 13.36 per cent, of corundum.'''' How 

 little Prol. Rammelsberg seems to appreciate the drift of the whole investiga- 

 tion, is proved by the tact that he attacks the purity of my mineral species, 

 when I never intended to publish these analyses as those of typical specimens 

 of spinel, but, on the contrary, as mixtures, still showing remnants of the origi- 

 nal species, and I distinctly say : " This analysis, however, establishes the very 

 important fact of the mechanical admixture of corundum.'''' 



On page 182, Prof. Rammelsberg says : " According to Hunt bauxite is changed 

 into corundum by strong ignition, and Genth thinks that this alteration takes 

 PLACE AT ORDINARY TEMPERATURE ALSO." Xow, the sccoud part of the Sentence 



