402 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Photographs 8 and 9. No. 8 indicates the result of etching (110) (crys- 

 tal face) with caustic soda. The pits are like those on the inner zones in 

 possessing a shelly structure, but have a different outline (compare 

 Photograph 9). 



Before leaving this peculiar hornblende, it should be stated that it 

 stands also in a unique position with respect to the etching properties of 

 the clinopinacoid. I have called the angle A D H \n Figure 8 posi- 

 tive ; it varies in value with the different varieties of aluminous amphi- 

 boles from 1° to 10". But, in the analogous pit produced on (010) of 

 the Philipstad hornblende, this angle is always negative and averages 2^° 

 in value. The description of the optical and other characters of the 

 mineral is deferred to another occasion. (See following article.) 



Etch-Hills on Hornblende^ (110). 



A digression from the main subject of types of pits of corrosion may 

 be permitted in the form of a short discussion of another result of attack 

 with hydrofluoric acid, namely, etch-hills. The usual effect of dissolving 

 a cleavage piece is, in time, the disappearance of the pits formed at the 

 beginning, and their replacement by these residual bosses. When they 

 are numerous, the mineral has a characteristic raammillated look. Besides 

 the normal bosses left on the removal of the ridges between successive 

 pits, however, there often appear on aluminous amphiboles, when etched 

 rapidly (fresh acid and high temperature), a variable number of remark- 

 able etch-hills which, from their form, can have but little to do with 

 those just mentioned. 



As examples, V. 46 furnishes some very striking specimens. I pro- 

 duced these peculiar etch-hills at 3 minutes, at '6\ minutes, at 7 minutes, 

 and also after 12 minutes' suspension in IIF gas evolved from a hot 

 (fuming) aqueous solution of that gas. In all four cases, the bosses were 

 on the whole similar in look, and, on account of their perfect development, 

 I shall describe the etch-hills on the cleavage piece last mentioned, as typ- 

 ical of all (Photograph 10). They are bizarre in form and arrangement; 

 in plan, triangles, irregular quadrilaterals of many shapes, trapezoids, 

 pentagons, etc., sometimes in groups of two, three, or a half-dozen, similar 

 to one another in outline, and even showing parallelism between cor- 

 responding sides. They are commonly bounded by straight lines that 

 have no definite relation to the hornblende crystal, and are thus in strik- 

 ing contrast to the pits which are oriented in the regular way on the 

 same cleavage face. Occasionally, small groups of the figures have the 

 same form and orientation ; thus, three scalene triangles were observed 



