84 E. tS. Dana — Choiidrodite from the Tilly-Foster Iron Mine. 



Crystals of simjyler habit. — The crystals thus far described have 

 all been of a more or less complicated character. But allusion has 

 been made to some very simple crystals, which also deserve descrip- 

 tion. The distinction is in most cases probably unimportant, 

 though lielieved to be of interest at first. The simple crystals are 

 uniformly large; they are so generally altered, and appear so differ- 

 ently from their small brilliant relatives, that it was supposed that 

 they differed from them at least in the purity of the original mate- 

 rial, if not more essentially. One brilliant exception, however, to the 

 general rule in regard to the altered condition of these crystals, in the 

 form of an isolated crystal of faultless luster, and deep red color, 

 about f of an inch in length, as well as numerous examples of transi- 

 tion products between the altered and unaltered material, made it 

 probable that all the crystals in question were originally of the same 

 character. Some examples are given in figures 3, 4 and 5. The 

 angles could be measured approximately only with a hand goniom- 

 eter, but there is no question that they, as well as others, belong 

 to type II. On one of them ±r was observed. 



In what has been said exception must be made in regard to the 

 large coarse crystals, and crystalline masses, mentioned in the early 

 part of this article, and which are made up of a more or less hetero- 

 geneous mass of chondrodite, magnetite and sometimes rijjidolite ; 

 some, at least, of these last, belong to type I. (See p. 25.) 



Twins. — The humite crystals of Vesuvius, as well as the Swedish 

 chondrodite, has been shown by vom Rath to possess so great a ten- 

 dency to twinning that it is a little remarkable that the contrary 

 should be true of the mineral from Brewster. Figs. 20, 21, show the 

 only method of twinning which has been found, as well as the only 

 distinct twin-crystal. The axis of revolution here is the vertical axis 

 of the crystal, and the composition-face the basal plane A. Unfor 

 tunately the crystal in question was quite imperfect, and all that was 

 available is shown in the figure. The plane H (/-?=iOO) gave no 

 reflections, so that all measurements were made on e^(that is 201 

 and 201) ; in this case these planes were similar in luster as a result of 

 the twinning. A revolution of the kind mentioned (in a perfectly 

 symmetrical crystal) would, so far as this half of the crystal goes, have 

 the effect only of making it holohedral, giving no re-entrant angles ; 

 but, in case of any irregularity, it might give, as here, a re-entrant 

 angle in the planes which are hemihedral in their occurrence. 



The measurement of the re-entrant angle here observed gave for 

 *w2^to2, 10° 38' and 10° 40'; required 10° 39'. The other angles 

 measured on the same crystal are given in the following table. 



