12 MS. Dana — Chondrodite from the Tilhj-Foster Iron Mine. 



attempt to correct them was deemed desirable. Calculated from 

 these angles the parameters are : 



a (vert.)=l-57236 ; ^'^l; c=l-08630; 



and the angle for the fundamental prism is 



J/s,Z(llO/sllO)==85° 15' 46" or 94° 44' 14". 



It may be added that the angle of A on / behind (siY) was meas- 

 ured with equal care, and found to be 135° 18' 40". 



The preceding table* (I) includes the principal angles measured on 

 the same crystal, and also those calculated from the above parame- 

 ters ; in addition, the corresponding angles for humite, type II, are 

 also given, as calculated by vom Rath. The angles of the maci-o- 

 domes agree very closely, it will be observed, in chondrodite and 

 humite ; in the brachydomes, on the other hand, there is a divergence 

 of 6 or 7 minutes. 



The angles given in table I, and also in tables XII, XIII, XIV, 

 for types I and III, are the actual angles. In all the other tables, 

 however, the supplement (normal) angles, as measured and calculated, 

 are uniformly given. The reason for this was that these angles hav- 

 ing special reference to the sphere of projection, and being chiefly of 

 value in calculating with it, it did not seem worth while to change 

 them from the form in which they had been used. 



It is necessary to explain, also, the system of symbols and let- 

 ters here and subsequently employed. The fundamental foi-m adopted 

 is the same as that used in Dana's " Mineralogy," and first suggested 

 by the author of that work in Am. J. Sci. II, xiv, 175, 185. It is to 

 be remembered that Scacchi made the prism of the r series (i. e., «-3) 

 the fundamental prism, and gave to the vertical axes lengths, in the ratio 

 of 7 : 5 : 9 to each other, for types I, II and III respectively. Vom 

 Rath followed him in this latter respect, but for the vertical prism 

 took that of the 7i series (i. e., 7, or 110 of Dana). Prof Dana took a 

 modified view of the relations of the three forms, and chose for 

 the fundamental macrodome in each type the plane making with 

 A{ 0) an angle of 122° to 125° : thus on humite (vom Rath), type I, 

 124° 17', II, 122° 28', III, 125° 15'. In this view the vertical axes 

 have one-third (I), one-half (TI) and one-quarter (III) of the lengths 

 assumed by Scacchi ; in other words, their relation to each other are 



* Both the symbols of Naiimann (in the form used in Dana's "Mineralogy") and also 

 of Miller are given; the signs belonging to each plane omitted Jiere, as the relations of 

 the planes are shown with sufficient clearness on the spherical projection, Plate xiii. 



