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PROCEEDINGS OP THE AMERICAN ACADEMY. 



discoverable rule governing its development, and I even doubt that it 

 is always the same face. We might suppose that B G is composed of a 

 large number of minute planes, and that, for some at present unknown 

 reason, there is a selection of now one, now another, of these multitu- 

 dinous facets, which grows rapidly and becomes prominent as a fourth 

 figure-face of the pit. 



Pargasite, carinthine, gamsigradite, syntagmatite, and barkevikite (see 

 Plate I. Fig. 20), each from its classic locality, give figures that are equiv- 

 alent with those just described for basaltic hornblende. The differences, 

 if any, from the Wolfsberg sub-type are so slight as to prevent any 

 determination of these varieties from the shapes of their respective 

 corrosion pits. The other three sub-types merit a few remarks, inasmuch 

 as they undoubtedly owe their distinctive characters primarily to chemi- 

 cal composition. 



Figure 3. 



Tlie Kragero Sub-type, (110). 



The pits on (110) of V. 31 were sotne- 

 what sharper than those usually obtainable 

 on basaltic hornblejide (Figure 3, Plate I. 

 Figs. 9 and 10, and Photograph 2). There 

 are again normally three, abnormally four, 

 figure-faces all oriented in the sense of the 

 corresponding faces of the pits of the Wolfs- 

 berg sub-type. The straightness of the edge 

 A D \% here characteristic. The angle D A H 

 is extraordinarily variable, having the value 

 of about 38° in the initial figures, aud in- 

 termediate values up to 80° in the large 

 matured pits. There is also a swivelling of 

 the edge A! , jy, but its amount is difiicult to 

 determine. Accompanying these changes in 

 the figure-faces, there is a tendency for the 

 figure to broaden out as it deepens; so that, 

 while in the initial figures the ratio of length 

 to breadth is about 3.5 : 1 (0.05 mm. : 0.014 

 mm.), that ratio is 2 : 1 (0.072 mm. : 0.03(> 

 mm.) in the matured pit. The angle C A H 

 ranges through limits about equal to the 

 Wolfsberfj readings. 



