276 B. K. Emerson— The Deerfelr/ Dyke one] iis Mrnmils. 



Products of the decomposition of Prehnite. Chlorophce- 

 tie (of Hitchcock.)— The mineral, chlorophieite, described by 

 Macculloch in 1825, but not analyzed, proves now, from the an- 

 alysis of Heddle,* to be of very different composition from the 

 highly hydrated protoxide of iron silicate analyzed by Forch- 

 hammer, with which it has been associated. It is a magnesian 

 peroxide of iron silicate with about 25 percent of water, some- 

 times aluminous, and it approaches thus much more nearly to 

 diabantite. from which it dill'ers mainly in the peroxidation of 

 the iron and in containing double the quantity of water. 



In 1825 Pres. Hitchcock discovered a mineral "in the trap 

 rocks about Turner's Falls, in Gill, Mass.,f which Professor J. 

 W. Webster, of Harvard I' Diversity, pronounced to be the 

 chlorophasite of Macculloch," and the description given by 

 Pres. Hitchcock agrees so exactly with that of Macculloch and 

 Heddle, and the rapid blackening of the mineral is so peculiar, 

 that the reference was very natural, and, 1 think, correct. 



On examining slides of the "smoked" prehnite last de- 

 scribed, its radiated needles were in the center colorless and 

 perfectly fresh, and contained in abundance scales of diabantite 

 with remarkably strong dichroism, brown-green to black. 

 Although the smoky black mass resembled closely an amyg- 

 dule of the chlorophieite its great hardness and the crystalline 

 surface 1 proved it to lie prehnite, and the peculiar striatioti de- 

 pendent upon the arrow-head twinning was quite as charac- 

 teristic. Toward the outer surface, however, the mass softened 

 rapidly and could he readily impressed by the finger-nail, and 

 under the microscope the fibers were seen to change quite 

 rapidly into an amorphous or finely scalv material of red- 

 brown color, showing faint aggregate polarization, but no 

 dichroism. Farther in among the clear needles portions of the 

 mass have all the needles likewise stained brown superficially. 

 On examining the original specimens of President Hitchcock, 

 No. 91, of the last catalogue of the Massachusetts State col- 

 lection,^ it was found to be distinctly radiated from several 

 centers, the fibers of the s mie size and arrangement as those of 

 the fine fibrous prehnite described ante p. (275), and a frag- 

 ment broken from the specimen gave a pale-green powder and 

 scratched apatite without difficulty. The cavity in which it is 

 found is lined first with the foliated diabantite, then follows 

 inwardly chaleopyrite, cldoroplueite, the latter occupying thus 

 the same place as the prehnite in the unaltered " nodules. 

 Under the microscope traces of the bright quartz-like polar- 



Report, vi, Appendix, p. lvi, where ti 



