83 



found, and which has therefore exclusively been the subject of 

 these examinations. 



Where the calcite is dissolved, the mentioned crystals 

 have always quite rough and dull faces, very often covered with 

 a crust of limonite, and therefore they appear under the micro- 

 scope quite irregular. In a few crystals we see, however, that 

 the faces either not at all, or only to a small degree, have been 

 exposed to disintegration, and in such faces we may then find 

 well developed etching figures. That they have been really 

 produced by an etching may be judged from the fact that it 

 has not been possible, even by a protracted search, to find 

 corresponding figures on the crystals still surrounded by the 

 calcite. 



Only in two different faces distinct natural etching figures 

 have been found, viz. the faces s{l20} and P{lOl}. 



s {120} is of peculiar interest as being the only face, in 

 which natural etching figures are found, and on which such 

 figures may be produced by acids ^^^^^ 



and alkalies. The two latter forms, as 

 we have seen, were as different as 

 only possible. It is now seen that 

 the natural etching figures occupy a 

 position midway between both, being 

 generally lengthened neither perpen- 

 dicularly nor horizontally. 



(Fig. 30) The form is commonly 

 as shown in the figure, but may in 

 a few cases be somewhat lengthened 

 perpendicularly, by which feature it 

 approaches more to the figures pro- 

 duced by acids, but with the difference 

 that the curved side looks towards the 

 edge formed with è(oio), while the 

 opposite side is constantly rectilinear. 



(301b 



(210) 







(30Î) 



(111) 



(ort) 



Fig. 30. Natural etching 

 figures on {120}. 



6* 



