﻿E. S. Dana — Mineralogical Notes. 387 



As fundamental angles the following (indicated by an asterisk 

 in the table) were accepted : 



130^130 = 136° 10' and 111 ^ III = 108° 43' 



The axial ratio deduced from these is given below, and the 

 above table contains the calculated angles : 



a : 6 : c = 0-82850 : 1 : 0-88976 (Dana's position). 

 a:b:c= 0*40234 : 1 : 0-35198 (Schrauf's position). 



A comparison of the measured and calculated angles shows a 

 very satisfactory agreement considering the character of the 

 species and the results previously obtained. Measurements on 

 a number of other less perfect crystals gave confirmatory 

 results. 



No reason was found for believing that there is any devia- 

 tion from the orthorhombic form such as has been often sug- 

 gested, and in fact it was proved that no deviation greater than 

 two or three minutes could exist. Arzruni and Grroth have 

 shown on optical grounds that columbite is really orthorhom- 

 bic. The axial ratio here deduced agrees much better with 

 that of J. D. Dana (in fact the angles of the prismatic zone are 

 nearly identical) than with that of Schrauf. The similarity in 

 angles between the Middletown and Standish crystals goes to 

 prove that the variation of form connected with the change of 

 composition must be small. The Middletown columbite con- 

 tains a large amount of iron tantalate (nearly 30 per cent), while 

 the Standish mineral is nearly pure iron columbate (niobate) 

 with the specific gravity =5'65. As bearing in the same 

 direction the writer found that the Northfield columbite, which 

 is almost a tantalite (57 per cent Ta 2 53 sp. grav. =6*8), had 

 also the habit and angles of ordinary columbite. 



The crystals of columbite from Standish resemble those of 

 Haddam and Middletown in habit, 

 though occasionally they are short 

 and stout, somewhat elongated in the 

 direction of the a axis and hence 

 approximate to the Greenland form. 

 The accompanying figure is a basal 

 projection of a common type. Twins 

 are common and all after the usual 

 law, the twinning plane the brachy- 

 dome e. They are generally the fam- 

 iliar heart-shaped contact twins, but much variation in habit is 

 observed. The observed planes on the Standish crystals are: 



a (100, i-l), b (010, i-%), c (001, (9), z (530, a-|), ' m (110, /), 

 g (130, t-3), I (106, H), & (103, f£), e (021, 2-S), o (111, 1), 

 fi (233, 1-f), u (133, 1-3), n (163, 2-6). 



