DALY. ■ 



ETCH-FIGDRES ON AMPHIBOLES. 



397 



and not yet shown to exist. The interrupted phases of solution are 

 more reasonably connected with the lamellation of the cleavage parallel 

 to the plane attacked. Actual microscopic or submicroscopic separation 

 of layers parallel to this cleavage would present to the solvent action 

 of the hydrofluoric acid a series of thin plates each of which, strongly 

 resistant in a direction at right angles to the broad flat surface, would 

 readily yield, in directions in that surface, i. e. along the grain of the 

 mineral. 



The above mentioned data regarding the highly important Wolfsberg 

 sub-type refer to conditions of etching described as standard for the 

 present investigation. Remembering Bomer's conclusion that the tem- 

 perature of the solvent has, in tlie case of HF and quartz, an influence 

 on the figures, I have recorded the facts from a number of experiments 

 intended to test the principle in its application to the group of the com- 

 mon and basaltic hornblendes. The experiments were made on V. 4G, 

 as follows : — 



(11) Room temperature 



16 hours 



Result. 

 Many poorly defined pits. 

 Larger poorly defined pits. 

 Small sliarply defined pits. 



Pits numerous, in every 

 case, some of great sharp- 

 ness and large enough to 

 measure. 



Pits indistinguishable from 

 the last, but associated 

 with etch-hills. 



Few but good figures of the 

 foregoing type. 



Throughout the whole series there is such a close correspondence in the 

 forms and measurements of the figures that we must posit for the latter 

 an independence of the temperature of the hydrofluoric acid, so far as 

 any sensible differences are concerned. 



Neither temperature nor concentration of acid, nor, indeed, any cause 

 known to me will explain a notable variation in the shape of the pits on 

 (110) of the amphiboles now under consideration. It consists in the 

 appearance of an adventitious fourth figure-face on the southeast side, 

 in addition to the three usual ones (see Plate I. Fig. 8). There is no 



