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



which can be cut out with a knife. These crystals vary in size, being 

 sometimes an inch or two in length. A crystallographic examination 

 of them is seldom possible, but a few of the crystals found allow of 

 it, and the results are described beyond. The form is usually very 

 simple, and the color varies from a deep red to a light yellow. This 

 may be said to be the common method of occurrence at the locality. 

 Forti;nately, matei'ial much l)etter adapted for crystallographic 

 study also occurs, though this is very rarely true. Narrow veins 

 are sometimes met with, two or three inches across, which were 

 originally lined with more or less perfectly crystallized chondrodite 

 and also with dodecahedrons of magnetite, crystals of rijDidolite, and 

 rarely apatite, and then subsequently filled in with dolomite. Where 

 this has been the case and the dolomite has remained intact the 

 chondrodite has been protected and the crystals have retained per- 

 fectly their luster and color. Only in a few instances were the ciys- 

 tals polished when covered simply by a soft serpentine. The chem- 

 ical composition of this chondrodite is given beyond, after the 

 description of the crystals. It has a deep, gai'net-red color, and 

 a luster equal to that of the finest Binnenthal blende. 



1. Description of Crystals belonging to Type II. 



The remark of vom Rath in regard to the irregularity of form of 

 the Swedish chondrodite is eminently true of the Bi'ewster crystals. 

 For in the same little group no two are alike ; so that each one de- 

 serves and requires an especial study. 



The first point to be determined was the values of the fundamental 

 angles. Some difficulty was found in obtaining these from the fact 

 that many crystals, though faultless in luster, yet gave uncertain 

 measurements. This was due to the fractured condition of many of 

 the planes, which, though often not very apparent at first sight, yet 

 gave rise to a variety of reflected images in the goniometer, of which 

 no one could be accepted as trustworthy. All the larger crystals 

 show a multitude of internal fractures; and, where such crystals 

 have been subjected to altering influences, this circumstance has has- 

 tened their destruction, and in all cases the external condition of the 

 planes has been more or less aflected. The direction of the fracture 

 lines was in most cases entirely irregular, though in a number of 

 cases they were distinctly parallel to e^{-l-i). The presence of these 

 cracks gave the crystals the appearance of having suffered sudden 

 contraction, by which the planes had been irregularly drawn inward, 

 forming re-entrant angles ; in fact, in this respect, as in general 

 appearance, this chondrodite might be aptly compared to a resin. 



