376 PROCEEDINGS OF THE AMERICAN ACADEMY. 



apparently curved, faces. Many pits normally exhibit a figure-face 

 parallel to the plane etched ; it may be designated the " bottom-face." 

 An " etch-zone " is a zone containing two or more figure-faces (" Aetz- 

 zone" of Molengraaf, not equivalent with the " Aetzzone " of Becke). 

 The periphery of the pit where its figure-faces intersect the plane at- 

 tacked is here called the " outline " of the figure-, and each " edge " that 

 composes it may thus be a straight or curved line as its figure-face is 

 plane or curved. A " corner " is the point where the etched plane and 

 two adjacent figure-faces meet. 



As the process of etching continues, a pit usually increases in size, 

 often (depending partly on the symmetry of the etched plane) changes in 

 shape of outline, and, in many cases, deepens as the result of replace- 

 ment of early formed figure-faces by others of different indices, accom- 

 panied by the necessary " diminishing " of the " bottom-face " if there be 

 one present (cf. pits on apatite, calcite, galenite, gypsum, zinc-blende, 

 etc.). These changes in the figures may be continuous, but often have 

 rather the look of being intermittent, the replacement of one figure-face 

 by another taking place as a momentary cliange, faces of intermediate 

 indices not appearing at all. The first stage of development of a pit 

 may be called its " initial " form. The development ends where the out- 

 line besfins to be seriously impaired by the solution of the surrounding 

 part of the etched surface. Just preceding this point in the history, the 

 pit may be called " mature," and the process intervening between the 

 initial and mature stages is that of " maturing." Von Ebner's " instan- 

 taneous " and " retarded " types are connected by transitions, but are 

 not easily to be compared to " initial " and " mature " figures, since his 

 types refer simply to the length of time required to develop the pits, and 

 are not restricted to the use of one solvent.* 



Discussion of Etch-Figures in the Microscope. 



It is believed that a description of the etch-figures as seen in reflected 

 light with vertical incidence would be, on the whole, of more value than 

 an account of the same figures examined under other conditions (trans- 

 mitted light, Lichtschimmer). Within certain limits, this method is 

 easily carried out with the aid of the modern appliances to be found on 

 the large models of most petrographical microscopes, and thus a new 

 etch-figure can in a few minutes be compared in its main features with 



* Sitzungsber. der Akad. d. Wissen., Vienna, 1885, Bd. XCI. p. 776 



