494 



TRANS. ST. I.OUIS ACAD. SCIENCE. 



The Milleiite occurs in a variety of forms. The purest crys- 

 talline variety forms thin, long, needle-like prisms, many of them 

 uniting in one point at the wall of the cavity or on calcite-crys- 

 tals, and radiating from this point into the cavity. They are i-6" 

 long, very elastic, of high metallic lustre, and brass-yellow color. 

 Under the microscope these " needles" show a six-sided prism, 

 striped longitudinally and terminated by the rhombohedral faces. 

 Those examined by me varied in thickness from 0.03-0.05 mm. 

 If pressed with a hard instrument these prisms divided into nu- 

 merous fibres, each of which showed the same prismatic form. 

 The thickness of these fibres was 0.0007-0.0015 mm. The angle 

 "between the face of the rhombohedron and the prism — average 

 of many measurements— is 110° 36'. These crystals are often 

 enclosed in crystals of calcite, sometimes in such amounts as to 

 render the calcite opaque. 



The Millerite is also found in aggregations, up to 2 inches long 

 and to i inch thick, formed by many prisms adhering to each 

 other. These spear-like forms are usually round, sometimes flat- 

 tened, and pointed at both ends. They occasionally traverse 

 crystals of Calcite, or are enclosed at one end in crystals of Flu- 

 orite. These forms have not the high metallic lustre of the 

 needle-like crystals, but are mostly dull and of darker color. 



Most frequently the Millerite appears in very thin thread-like 

 forms which are irregularly bent and interwoven. The thickness 

 of these prisms is the same as of those fibres composing the 

 straight needle-like prisms. The thin threads, of dark green color, 

 are often so closely interwoven as to resemble very thick spider- 

 web, and, in that form, lie over crystals of Fluorite, Calcite, or 

 Dolomite. More frequently they form bunches looking like 

 bunches of hair, which sometimes fill the whole interior of cavi- 

 ties that are lined with some of the other minerals. Small crys- 

 tals of Dolomite are often attached to these forms ; also little 

 rounded aggregations of Barite and Strontianite, which are al- 

 ways in an advanced state of decomposition, snowy-white, crum- 

 bling between the fingers. 



Minute crystals of Pyrite rarely surround these prisms of Mil- 

 lerite. The Pyrite as seen under the microscope is in cubical 

 form, and shows plainly the striation of the faces. The Millerite 



