EXPLANATION OF PLATE 22. 



Microscopic Thin Sections of Rocks, Used for Aboriginal Implements. 



Fig. 1. Quartz pokphyry. A dense felsite groundmass, bearing abundant quartzes 

 in botli rounded and angular forms, often deeply corroded and more rarely 

 well-detined pbenocrysts. Tbt* structure is common to tbe quartz porpby- 

 ries, but sbows no appreciable flow structure. In a single instance is noted 

 a brilliantly polarizing aggregate of tbe manganese epidote, piedmoutite. 

 (Cat. No. 27861, r .S.N.M. Norfolk, Connecticut.) 



Fig. 2. ARciiLLiTK ?. Schistose, semi-metamorphic rock, the optical properties of 

 which are too obscure for satisfactory determination. 

 ((C. B.) Cliain Bridge, Virginia, or Di.strict of Coluniliia.) 

 Fig. 3. DiAHASE This shows a wholly crystalline aggregate of elongated feldspar 

 and augite with the characteristic ophitic structure of diabase. 

 (Cat. No. 16708, U.S.N.M. Spartanburg, South Carolina.) 



Mineralogical lU'scriptions by Prof. (i. P. Merrill, V. S. National Museum. 



