Schaller — Siderite and Barite from Maryland. 367 



Etch Figures. — A cleavage piece was left standing in cold 

 dilate hydrochloric acid for several days and then examined 

 under the microscope, when well-defined etch figures conld "be 

 observed. These are triangular in shape and possess a plane 

 of symmetry parallel with the shorter diagonal of the cleavage 

 rhomb of siderite, and are shown 2 



in figure 2. They resemble in 

 symmetry the figures shown in 

 Miers' Mineralogy (page 112) for 

 calcite and not those given for 

 dolomite. The symmetry of sid- 

 erite is, therefore, the same as 

 that of calcite and not that of 

 dolomite, a conclusion sustained 



by the forms of the crystals. Et( * 4§ u ^ s on cleava S e sur " 



J T7- 7 j? • v ' i ill. i face of siderite. 



Value for c-axis. — Although 



siderite is a common mineral, good crystals are rare and the 



literature on siderite is poor in crystallographic data. The 



only value for the axial ratio given and which is adopted in all 



books is one obtained in 1812 by Wollaston. His statement 



in regard to siderite is as follows :* "I have examined various 



specimens of this substance, some pure white, others brown, 



some transparent, others opake. That which gives the most 



distinct image by reflection is of a brownish hue, with the 



semi-transparency of horn. It was obtained from a tin mine, 



called Maudlin Mine, near Lostwithiel in Cornwall. By 



repeated measurements of small fragments of this specimen, the 



angle appears to be so nearly 107°, that I cannot form any 



judgment whether in perfect crystals it will prove to be greater 



or less than that angle. 



In this instance the carbonate of iron is nearly pure, and so 

 perfectly free from carbonate of lime " 



The measurements of the faces giving good reflections were 

 used for calculating a value for the c-axis. The angles for the 

 same form varied somewhat on different crystals, though the 

 values obtained from the measurements of different forms agree 

 very well with each other. From the average reading, the 

 following values were calculated : 



From 10 meas. of r = \1011\, c = -82352 

 " 27 " " v = {2131}, c — -82463 

 " 16 " " y = {3251}, c = -82311 



, Average, c = '8240 



An attempt was made to measure the cleavage angle directly, 

 but it was found that the resultant cleavage faces were never 



*Phil. Trans., 159, 1812. 



