E. S. Dana — Chondrodite from the Tilly-Foster Iroti 3Ime. 69 



447, 1874. It may be of interest, liowever, to review tlie subject again 

 so far as the chondroilite itself is immediately concerned. 



The chondrodite forms the gangue of the magnetite, being every- 

 where disseminated through it in varying [troportions. In tlie parts 

 of the mine wliere the ore is purest and perfectly firm and solid, — the 

 so-called " blue ore," — the associated chondrodite is sparsely sprin- 

 kled through it in small yellow grains, showing no trace of crystal- 

 line form. Occasionally, however, the firmer ore contains the chon- 

 drodite in very large but im})erfect crystals, or crystalline masses, 

 associated directly with enstatite and enveloped with dolomite, which 

 have a dark, rich brown color, and a brilliant luster on the fracture. 

 A distinctly laminated structure is uniformly ])resent, which is per- 

 haps due to cleavage (?) (See page 21.) Isolated grains imbedded in 

 dolomite often show traces of crystalline faces, though nothing that 

 admits of even approximate determination. An analysis of this vari- 

 ety of the mineral gave Mr. Breidenbaugh (Am. J. Sci., Ill, vi, 209), 



Si 35-42, Fe 5-72, Mg 54-22, Fl 9-00 = 104-3G; equivalent of oxygen replaced by- 

 fluorine, 3-79. 



In the lai'ger portion of the mine as no-w opened the soft " yellow 

 ore" predominates : the chondrodite is present in it in much larger 

 quantities, and, like the other minerals present, it has almost uni- 

 versally suiFered extensive alteration. A long list of these products 

 of alteration has been fully described by Prof. Dana in the memoir 

 already alluded to. The chondrodite forms the main portion of the 

 material taken out, and many tons of this refuse matter are yearly 

 thrown away. It vai'ies much in color, but is generally of a light 

 yellow; it iisually has more or less of a soapy feel and shows a vari- 

 ety of transition-products between the semi-altered material and 

 serpentine. The chondrodite in this " yellow ore" is generally mas- 

 sive ; but occasionally fragments of large coarse crystals have been 

 found, some of which measure five or six inches in length. These 

 are always more or less altered ; moreover, the material of which 

 they are formed is far from homogeneous, masses of magnetite, and 

 also chlorite, being often enclosed. Dolomite is the most constantly 

 associated mineral and occurs in rhombohedrons of considerable 

 size ; these, as well as the crystals of chondrodite, are often coated 

 ■with magnetite. 



Better crystals of chondrodite than those just mentioned are some- 

 times found in what Avere once cavities in this massive material. 

 Unfortunately these have all suffered from the general alteration and 

 now have little or no luster, and often are not even smooth. These 

 cavities are almost invariably filled with a soft mealy serpentine, 



