344 C. JI. Warren — Mineralogical Notes. 



ity of 6'329 was calculated for a carbonate having the same 

 composition as that of the one analyzed. This compares very 

 satisfactorily with the actual specific gravity, 6409. 



Associated with the cerussite is the chlor-carbonate of lead, 

 phosgenite. This is distinguished in appearance from the 

 cerussite by its clear brown color and by three excellent cleav- 

 ages, prismatic and basal, at right angles to each other. It was 

 possible to identify the basal cleavage by the positive uniaxial 

 interference figure obtained when sections parallel to this 

 cleavage were examined under the microscope. On such frag- 

 ments a much poorer cleavage approximately half way between 

 the prismatic cleavages was also observed, indicating the pres- 

 ence of the cleavage parallel to the face 100. The prismatic 

 and basal cleavages are of about the same degree of perfection. 

 In one specimen a somewhat tabular habit was noticed parallel 

 to the basal cleavage. 



The relative amounts of cerussite and phosgenite vary con- 

 siderably in different specimens, but the latter has always been 

 observed as a core surrounded by the former. In one specimen, 

 weighing nearly two pounds, the cerussite is simply a rim aver- 

 aging V^"^ in thickness. This is separated from the phosgenite 

 by a very narrow white band of powdery material. The above 

 facts suggest that the cerussite is an alteration product of the 

 phosgenite. Small cavities, possibly formed by solution, lined 

 with minute acicular crystallizations of cerussite have been 

 noticed on most of the specimens examined. 



Contributions from the Geological Department of the Massachusetts Insti- 

 tute of Technology, No. 116, vol. 15. 

 Boston, Mass., June, 1903. 



