DARK SCALE OF HARDNESS 725 



has been recognized and coal-bearing deposits of terrestrial origin. The lat- 

 ter possibly include beds transitional into the Tertiary. 



DARK SCALE OF HARDNESS 

 BY ALFRED C. LANE 



(A'bstract) 



The hardness of a mineral is its resistance to shearing stress. Like other 

 properties of minerals, it may differ in different directions. When two similar 

 surfaces are rubbed together, the softer mineral leaves a powder (streak) on 

 the other. In order to be sure which mineral gives the streak, it is at times 

 convenient to have besides the common Mobs scale of hardness, composed of 

 light minerals, a "dark scale of hardness" of minerals whose color and streak 

 are dark, especially in teaching. For such minerals the following properties 

 are desirable : quickly recognizable, easily obtainable, hardness uniform. 



The following minerals have been used by the writer: (1) Graphite, with 

 one good cleavage, at one extreme in the white scale ; at the other extreme in 

 the black. (2) Stibnite (Sb.Ss), with two good cleavages, bladed. (3) Galen- 

 ite (PbSa), with three good cleavages. (4) Iron (use soft wire nail), mag- 

 netic, ductile. (5) Niccolite (NiAs), characterii-tic color, no cleavage, (fj) 

 Magnetite (FegOJ, magnetic, brittle. 



The minerals mentioned seem fairly satisfactory. Above 6 the author does 

 not know whether spinel or some other mineral would be more desirable, but 

 as there are comparatively few minerals concerned, it is not important. 



DEMONSTRATION OF RELATIVE REFRACTION 

 BY ALFRED C. LANE 



(Abstract) 



The method of determining relative index of refraction developed by Exner, 

 Becker, and Schroeder van der Kolk^ may be demonstrated to a class as fol- 

 lows : A large beaker of water placed just a little to one side of a window or 

 other source of light will show a bright streak on the farther side, having a 

 higher index than the surrounding medium. A test tube full of air placed in 

 the beaker will show a bright streak of total reflection on the nearer side, 

 the air within having a less index than the surrounding medium. 



STRATIGRAPIIIC STUDY OF THE APPALACHIAN AND CENTRAL STATES WITH 

 REFERENCE TO THE OCCURRENCE OF OIL AND GAS 



BY GEO. H. ASHLEY 



(Abstract) 



It had always been supposed by the oil and gas men that the Appalachian 

 region, extending from Pennsylvania to Alabama, was a stratigraphic unit, 

 and it has been a mystery that eastern Kentucky, Tennessee, and Alabama 

 should not yield as much oil and gas as Pennsylvania or West Virginia. The 



1 Report Micbigan Geological Survey, vol. vl, p. 154. 



