68 E. S. Dana — Chondrodite from the Tilly- Foster Iron Mine. 



The chemical composition of the three types of humite has been 

 most recently investigated by vom Rath,* and although analyses lead 

 to somewhat different results in the three cases, he concludes that in 

 composition they are still essentially the same, and that the cause of 

 the variation in crystalline form is not to be found in the relative 

 amount of fluorine present, as has been often assumed. 



A further remarkable peculiarity true of two of the three types is 

 their hemihcdral character, which is clearly set forth in the memoirs 

 referred to. These points are alluded to here because of their direct 

 bearing on the crystallization of chondrodite, which forms the sub- 

 ject of this j^aper, 



Chondrodite was first shown by Rammelsberg to be identical with 

 humite in chemical composition, but its ci-ystallographic relation to it 

 was not brought out until the investigations of Kokscharow. He 

 showed, in his " Materialien zur Mineralogie Russlands," vol. vi, p. 73, 

 1870, that the crystals from Pargas, Finland, were identical in form 

 and angles with type II of humite. Vom Rath has followed with the 

 description of crystals from Nya-Kopparberg, Sweden, and px'oved 

 that the same fact is true of them. 



The study of the chondrodite from the Tilly-Foster iron mine, 

 Brewster, Putnam Co., New York, which I have been able to make 

 during the past season, has shown that it, too, is for the most part iden- 

 tical in crystalline form with type II of humite, but that at the same 

 time crystals exist belonging to type I, and others which belong to 

 type III. Further than this, the chemical composition of the second 

 type crystals, as shown by an analysis by Mr. G. W. Hawes (p. 21), 

 agrees with great exactness with that of the Swedish mineral anal- 

 yzed by vom Rath. Moreover, the detailed study of these crystals 

 has shown that while they agree with humite in the character of 

 their hemihedrism, as well as in angles, they surpass it in the multi- 

 plicity of secondary planes. Thus a single solid angle has been 

 observed M^hich was modified by fifteen distinct and well-defined, 

 though very minute, planes. This, as will be seen when the facts 

 are described in detail, implies a delicacy in the action of the 

 crystallogenic forces at work which is unparalleled, and sustains the 

 opinion that chondrodite, or humite, is unique among mineral species. 



The method of occurrence at the Tilly-Foster iron mine has been 

 fully described by Prof. Dana in a memoir entitled, " Serpentine 

 pseudomorphs and other kinds, etc.," Journal of Science, viii, pp. 371, 



* Pogg. Ann., cxlvii, 246, 18'72. 



