RE-EXAMINATION OF AMERICAN MINERALS. 



163 



Fig. 2. 



tals are sometimes of a remarkable character, being eight or 

 ten inches in length and only a quarter of an inch in diameter, 

 preserving a tolerably perfect hexagonal shape throughout the 

 entire length ; again, these slender forms are built up of small 

 hexagonal prisms, their faces projecting from the side. It 

 sometimes happens that these slender crystals are crossed by 

 one of the same diameter and less length, firmly attached in 

 the manner of a cross. 



But of all remarkable crystallizations is one where the 

 small prisms are so arranged as to form a perfect double spiral 



arranged around an 

 axis (fig. 1). The spec- 

 imen is three inches 

 in length and three 

 eighths of an inch in 

 diameter, with the space Of one fourth of an inch between 

 each turn of the spiral. The spiral arises from one small 

 prism crossing another at middle at a small angle of diver- 

 gence (40°-50°), and so on in succession. These slender crys- 

 tals are sometimes curved in a very remarkable manner. 



Another thing to be remarked in connection with the calcite 

 of this mine is its singular associations; thus, we find groups 

 of hexagonal prisms where a small cubical 

 crystal of fluor, about the one twentieth of 

 an inch, is inserted in a small pit in the 

 summit of almost every crystal (figure 3) 

 without the occurrence of fluor-spar on any 

 other parts of the crystal. These crystals 

 appear to have been formed by successive 

 crystallizations. Dog-tooth spar seems to 

 have been first formed with these small 

 crystals of fluor-spar on their extremities, 

 and then by a subsequent process the calcite 

 has closed around the dog-tooth spar in the form of a hexagonal 

 prism with a three-sided summit. The summit never closes 

 entirely at the center, the fluor-spar remaining visible on one 

 side; and where there is no crystal of fluor-spar the extremity 

 of the dog-tooth spar is frequently seen. 



Other groups of calcite crystals have minute crystals of iron 

 pyrites in the three faces of the summit, arranged near and 



Fig. 



