86 E. S. Da7ia — Chondrodite front the Tilly- Foster Iron Mine. 



tion of the plane. It will be noticed that the four planes upon which 

 the inclination of * was measured are so situated, that any variation 

 from the true position in the index would sliow itself very conspicu- 

 ously. The fundamental angles for * are as follows : 



*/s^l(001)= 88° 51' 41'' 

 ^^^(lOO)^: 34 32 38 

 */s (7(010)r= 55 28 49 



In figure 16 a crystal, or portion of one, is exhibited which is holo- 

 hedral. It is irregular in this respect, however, that r^ forms a re- 

 entrant angle with r^. This is not a point of special importance, as an 

 irregularity such as this is often observed ; but, in view of the crystal 

 which has just been described, it is possible that here also there has 

 been a semi-revolution parallel to the basal plane. A more interest- 

 ing crystal, already once alluded to, is shown in fig. 4. It is con- 

 spicuously hemimorphic, as far as the form goes. It is large, and 

 admits only of approximate measurements, but there is no doubt but 

 that the planes as given have been determined correctly. In view of 

 the fact that a revolution parallel to would produce just the eifect 

 we have liere, and as such a twinning law has been shown to exist in 

 another conspicuous case, it is altogether probable that this forms an 

 ample explanation of what is observed. Another exactly parallel 

 case is noted under the description of two crystals of the 3d type. 

 The above described crystal was somewhat altei-ed, and so far imbedded 

 in the matrix that any experiments as to its pyro-electrical character 

 were out of the question. 



Chemical composition. — I am glad to be able to add here the re- 

 sults of a chemical examination of the chondrodite of the 2d type 

 from this locality, by Mr. G. \¥. Hawes of the Sheffield Scientific 

 School. It obviously increases much the value of this memoir. An 

 analysis by Mr. Breidenbaugh has already been quoted (p. 3). 



The material analyzed by Mr. Hawes consisted of fragments of 

 crystals of the 2d type, selected with great care to avoid the pres- 

 ence of any altered material. It had a deep garnet-red color and a 

 brilliant vitreous luster. Its specific gravity as determined by Mr. 



Hawes was 3-22. 



Analysis I. Analysis II. 



Silica 34-10 34-05 



Magnesia 53*17 53-72 



Ferrous oxide 7-17 7-28 



Alumina -48 "41 



Fluoi-ine 4-14 3-88 



99-06 99-34 



