50 Branner and Brackett — Peridotite of Arkansas. 



and normal silver. It is probable that normal silver, precipi- 

 tated in fine powder and set aside moist to dry gradually, may 

 cohere into brittle lumps, but these would be mere aggrega- 

 tions of discontinuous material. With allotropic silver the 

 case is very different, the particles dry in optical contact with 

 each other, the surfaces are brilliant and the material evidently 

 continuous. That this should be brittle indicates a totally 

 different state of molecular constitution from that of normal 

 silver. 



Specific Gravities. — The allotropic forms of silver show a 

 lower specific gravity than that of normal silver. 



In determining the specific gravities it was found essential to 

 keep the sp. gr. bottle after placing the material in it for some 

 hours under the bell of an air pump. Films of air attach 

 themselves obstinately to the surfaces and escape but slowly 

 even in vacuo. 



Taken with this precaution, the blue substance B gave spe- 

 cific gravity 9*58 and the yellow substance C, sp. gr. 8*51. 

 The specific gravity of normal silver, after melting was found 

 by G. Rose to be 10*5. That of finely divided silver obtained 

 by precipitation is stated to be 10*62.* 



I believe these determinations to be exact for the specimens 

 employed. But the condition of aggregation may not improb- 

 ably vary somewhat in different specimens. It seems however 

 clear that these forms of silver have a lower specific gravity 

 than the normal, and this is what would be expected. 



Chestnut Hill, Philadelphia, May, 1889. 



Aet. IY. — The Peridotite of Pike County, Arkansas', by 

 John C. Branner, State Geologist of Arkansas, and 

 Richard jST. Brackett, Chemist of the Geological Survey 

 of Arkansas. With Plate I. 



Part I, oy John C. Branner. 



Two and a half miles southeast of Murfreesboro in Pike 

 county, Arkansas, is a small exposure of peridotite whose posi- 

 tion and topographic features are shown in detail upon the 

 accompanying map (Plate I.) The entire exposure is about 

 2400 feet long by 1600 feet wide, and lies upon the middle 

 of the line between sections 21 and 28 of township 8 south, 

 range 25 west. 



From a geological standpoint this exposure is an important 

 one, for, small as it is, it offers a sug-gestion regarding the time 

 and character of the disturbing influences, which, about the 



♦Watts' Diet., orig. ed., v, 277. 



