HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 523 



III. Colors due to iron-rich silicate minerals : 



(1) Black. Basalt, coast of Ireland. 37610 



(2) Dark gray. Diabase, York, Pennsylvania. 37020 



(3) Red. Granite (color due to rod feldspar), Otter Creek, Mount Desert, 



Maine. 35923 



(4) Pink, green spotted. Gran i to (color due to pink feldspar and green 



epidote), Dedliain, Massachusetts. 263*6 



(5) Black. Aropkibolite (color due to dark hornblende), Baltimore 



County, Maryland. 26857 



(6) Dark green. Serpentine, Easton, Pennsylvania. 70109 



(7) Yellow. Serpentine, Moutville, New Jersey. 69198 



IV. Color due in part to transparency of mineral constituents, principally 



feldspars : 

 Dark greenish black. Norite, Keeseville, New York. 38744 



V. Colors due mainly to the physical conditiou of the various constituents and alack 



of metallic oxides or other coloring material. 



(1) White. Chalk. England. 



(2) White. Crystalline limestone, Loudoun Couuty, Virginia. 38568 



VI. Chaugein colors due to causes above enumerated. 



(1) From black to dull red-brown caused by oxidation of magnetite and 



iron rich silicates. Diabase. Lewistou, Maine. Two samples. 70661 



(2) From pure white to dull red, caused by oxidation of ferrous carbon- 



ates. White limestone. Newbury, Massachusetts. 73037 



(3) From dark gray to buff, caused by oxidation or pyrite and iron pro- 

 toxide. 



(4) From pink to dirty white, caused by a bleaching out and change in 



the physical condition of the feldspathic constituent (commence- 

 ment of kaolinizatiou). Granite. Washington County, Maine. 25873 



(5) From reddish brown to gray, caused by a removal of coloring matter 



(iron oxide) by decomposing organic matter. Sandstone. Mar- 

 quette, Michigan. 70659 



Luster as a property of rocks does not, owing to their complex char- 

 acter, possess the same value, as a characteristic, as among minerals. 

 Nevertheless, as there are certain of the more compact and homoge- 

 neous varieties which possess characteristic lusters, these may perhaps 

 be well shown here. The exhibit is arranged as follows : 



(1) Vitreous luster Quartzite ; Obsidian. 



(2) Greasy luster Serpentine. 



(3) Pearly luster Steatite. 



(4) Metallic luster Hematite. 



(5) Iridescent luster Coal. 



(6) Dull, no luster Chalk. 



The fracture or manner of breaking of any rock is dependent more 

 upon structure than chemical or mineralogical composition. Many line, 

 even-grained crystalline or fragmental rocks break with a smooth, even 

 surface and are spoken of as having a straight or even fracture; others, 

 and particularly the very compact fiae-graiued varieties, break with 

 shell-like concave and convex surfaces, and are said to have a conchoi- 

 dal fracture. Still others have splintery, hackly, shaly, friable, or pul- 

 verulent fractures. These characteristics, being of value in determin- 



