44 PHYSICAL AND CIIEMTCAL PEOPEETJES OF EOCKS 



stituents, but burn perfectly white when submitted to liigh 

 temperatures and yield volatile organic compounds. The fact 

 that serpentines so frequently contain small traces of chromium, 

 early gave rise to the opinion that it was to this element that 

 was due the characteristic green color of the mineral. The 

 writer has elsewhere^ described serpentines of a beautiful oil 

 yellow and deep green color which, however, contain not a 

 trace of chromium or manganese, but only iron, which in this 

 case is in combination as a silicate. (See p. 106.) 



These color characteristics are of greater importance than 

 may at first appear, particularly from an economic standpoint. 

 One of the first essentials in a rock designed for architectural 

 use should be permanency of color. Deleterious changes are 

 particularly liable to occur in stone taken from below the water 

 level, where, protected from oxidation, or from variations in 

 temperature. Certain of the Ohio sandstones are of a blue- 

 gray color below the water level, but buffi above, where the 

 included iron sulphides and protoxide carbonates have been 

 acted upon by oxidation. The student should early make 

 himself acquainted with these characteristics, as in the field it 

 is as a rule only the more or less weathered surfaces that pre- 

 sent themselves for inspection. This subject is again referred 

 to in the chapter on rock weathering. 



Lustre as a property of rocks does not, owing to their com- 

 plex nature, possess the same value as a determinative charac- 

 teristic as among minerals. Certain of the more compact and 

 homogeneous varieties possess lustres which may be described 

 as vitreous, greasy, pearly, metallic, or iridescent. The meaning 

 of such terms is sufficiently evident, and the subject need not 

 be further dwelt upon here. 



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

 more upon structure than upon chemical or mineralogical com- 

 position. Many fine and evenly grained crystalline or frag- 

 mental rocks break with smooth, even surfaces, and are described 

 as having a straight or even fracture. Others break with shell- 

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

 cJioidal fracture. Still others are splintery, hackly, or shaly, 

 words the meaning of which is suflSciently evident without their 

 being described in detail. 



1 On the Serpentine of Montville, New Jersey, Prce. IT. S. National 

 Museum, 1888, p. 105. 



