Hunt.] 108 [June 2, 



ened and disintegrated, while tli3 quartz was of course unchanged. 

 I have not yet been able to submit these materials to a chemical 

 examination. 



By the courtesy of the State Engineer, Mr. B. D. Frost, I was 

 enabled to get some data with regard to the extent of the decayed, 

 or as it was called by those in charge, the " demoralized " rock. The 

 softening and disintegration of the gneiss were found to be complete 

 for a distance of six hundred feet from the west portal, where the 

 floor of the tunnel is two hundred feet from the surface of the hill, 

 and were partial at one thousand feet from the entrance, where it is 

 two hundred and eighty feet below. 



Prof. James Hall, who examined this tunnel immediately after me, 

 and has detailed his observations in the Document already cited, 

 learned that at a distance of twelve hundred feet or more from the 

 western entrance, a bed of brown hematite was traversed in the tun- 

 nel, and he afterwards discovered the outcrop of this ore-bed on the 

 hillside above, where it is from four to six feet in thickness. This 

 would indicate that a partial decomposition of the strata extends 

 still deeper than mentioned above, inasmuch as this bed is probably, 

 like the similar ones mined farther southward, in Kent and Salis- 

 bury, Connecticut (where they occur in decomposed gneiss rock), the 

 result of an epigenic change of pyrites beds, as was long since pointed 

 out by Prof. C. U. Shepard. The evidence before us seems to justify 

 the conclusion that the whole of the feldspathic rocks of Hoosac 

 Mountain were at one time to a considerable depth from the surface 

 in a decayed and softened condition. The agencies which removed 

 this decomposed rock from the other parts of the mountain, however, 

 spared this portion at its western base, where it still remains, an 

 evidence of a process which has not since affected the exposed and 

 still undecayed portions of the similar rocks which form the surface 

 of the whole Mountain. 



Prof. J. D. Dana on the Alteration of Rocks. 

 By T. Sterry Hunt, LL.D., F.R.S. 



A note from Prof. Dana was read at the meeting of this Society in 

 November last, commenting on my remarks on the history of pseudo- 

 morphism, and its connection with the alteration of rocks. He has, 

 moreover, seen fit to reproduce his statements with some little varia- 

 tions, on two other occasions within the past year, in the American 

 Journal of Science, the last time in the month of February, in a 



