520 REPOKT OF NATIONAL MUSEUM, 1900. 



groundniass 5>tancl out in marked contrast. The poi-ph3n'ies are, as a 

 rule, hard, tough, and without rift or grain. They may be of several 

 shades of color, as green with splashes of white, or a paler green or 

 red with white specks, etc. Near Charlotte, North Carolina, there 

 occurs a cream-colored quartz porphyr^^ which is penetrated by long- 

 parallel streaks of a dead-black color. The stone, when cut across the 

 streaks, has a spotted appearance (hence the name Leopavdite)\ cut 

 parallel with the streaks it gives a dentritic or moss-like effect. 



Porphyry tinds a use more as an ornamental stone than as a stone 

 for personal adornment. 



PREHNITE. 



CHLORASTROLITE — ZONOCHLORITE. 



Prehnite occurs more commonljMiiassive, usually in reniform, botr}"- 

 oidal, globular, and stalactitic shapes. Hardness, 6.5; specitic gravity^ 

 2.8 to 2.95; luster, vitreous, occasionally pearly on certain surfaces. 

 Color, light green of several shades, passing into white and gray, the 

 color often fading on exposure. When cut and polished the darker 

 kinds resemble chrysoprase in color and luster. 



Chlorastrolite, from chloros (green), aster (star), and lithos (stone), 

 is an impure variety of prehnite, occurring in small rounded pebbles 

 obtained from the trap on the shores of Isle Royale, Lake Superior. 

 It is opaque, of a mottled green color, somewhat chat03^ant on the 

 rounded sides, and receives a high polish. 



Zonochlorite occurs on a small island off Neepigon Bay, Lake 

 Superior. Its green color and banded appearance has given' it its 

 name from zona, a band; chloros, green; and lithos, stone, 



Prehnite occurs as a secondary mineral in veins and cavities in the 

 more basic eruptive rocks as basalt and diabase; less often in the 

 crystalline rocks, granite, gneiss, and sj^enite. 



One hundred parts contain: Silica. -i-S.S; alumina, 24. S; lime, 27.1; 

 water, -1.3, 



PYRITE AND MARCASITE. 



Pyrite and marcasite are occasionally cut into squares, ovals, and 

 other shapes for use as settings for rings, scarf pins, trinkets, etc. The 

 two minerals contain the same elements in the same proportions — iron 

 46,7 and sulphur 53.3 parts per 100 — but differ in their physical prop- 

 erties, Pyrite has a hardness of 6.5; specific gravity, 5.2; crystalline 

 form, isometric; color, brass yellow, and is the kind more commonlj^ 

 used for ornaments. Marcasite has a hardness of 6; specific gravity, 

 4.8; crystalline form, orthorhombic; color, pale or gra^^ish yellow. 

 During the eighteenth century both pyrite and marcasite were cut 

 into facetted forms and extensively used for jewehy. 



