'2 1 6 JSoness — Etching Figures of the 



immersion of ten seconds in the boiling acid was amply 

 sufficient to etch the Iceland spar, it was found necessary 

 to boil the magnesite for nearly a half hour in a solution 

 of the same strength before suitable etchings were 

 obtained (fig 13). The etch figures are very similar in 

 shape and orientation to those of calcite ; the outer con- 

 tour of the magnesite forms is always a straight line, 

 while those of the calcite are slightly bulging; a face at 

 the bottom of the figures is common in both carbonates. 

 All figures are symmetrical to a vertical plane. 



Siderite, being more easily soluble, required a shorter 

 immersion in the acid, which had been used for the former 

 carbonates ; good figures were obtained after an immer- 

 sion of one minute, and while they are oriented as are the 

 calcite and magnesite figures they are much longer and 

 not so wide. The calcite figures are about as wide as long, 

 while the length of the siderite figures is four or five 

 times its width and the sides almost straight; very few 

 possess basal planes. 



Siderite figures, examined after 10 seconds' immersion 

 in dilute HC1, appear shorter and broader with a deep 

 basal groove extending almost the entire length of the 

 figure (see fig 14). At this stage the one-minute figure 

 and the 10-seconds figure are about the same width, 

 which indicates that solution is more rapid at first in a 

 direction at right angles to the c axis, and that the more 

 mature stage represents an elongation of the primitive 

 form. If solution be allowed to continue for several min- 

 utes the figure becomes rounded, and resembles in a small 

 degree the calcite figure. 



The cleavage fragments, having rather an irregular 

 surface, became grooved along certain cleavage lines, and 

 figures starting from these depressions very naturally 

 developed into asymmetric forms, due to the inclined 

 position of the basal plane. Occasionally these grooves 

 intersect, and, due to the solution eating out the inter- 

 vening space, large figures are produced, which are very 

 different from the ordinary type, but still they indicate 

 the symmetry of the face. 



The cleavage pieces of rhodochrosite were next investi- 

 gated. A solution of HC1 of the same strength as that 

 used for the three preceding carbonates, was brought to 

 act upon the more vitreous pieces of the mineral for 20 

 seconds. It was readily observed that rhodochrosite 



