738 REVIEWS 



separately described in some detail, and certain peculiarities possessed 

 by each are pointed out. The few scattered isolated deposits are 

 likewise described. 



In the Bryant district, which is the most southwesterly one, the 

 bauxite occurs in two distinct forms : {a) granitic bauxite, and {b) piso- 

 litic bauxite. The granitic bauxite forms the basal portions of the 

 beds, and in most cases rests immediately on a layer of kaolin, derived 

 by the ordinary processes of decay from the syenite. The bauxite is 

 spongy in texture with no trace of the pisolitic structure, but showing 

 partial traces of the granitic structure in which the individual feld- 

 spars are replaced by a porus skeleton of alumina. It is probable, 

 says Hayes, that this type of bauxite is in every case derived directly 

 from the syenite by the decomposition of the feldspar and the eleolite, 

 and the removal in solution of silica, lime and alkalis, the alumina 

 alone remaining of the original constituents. The pisolitic type is 

 more uniform than that of the Georgia-Alabama region and forms the 

 upper parts of the beds. The two forms of ore are not separated, as 

 a rule, in the same section, by any sharp and definite line. 



In the Fourche Mountain district only the pisolitic type of the 

 ore has been found, which, when nearest the syenite margin, rests on 

 a layer of kaolin as in the Bryant district. Those deposits more dis- 

 tant from the syenite margin are probably interstratified with sedi- 

 mentary beds of Tertiary age. 



The scattered isolated deposits found between the two districts 

 resemble in their mode of occurrence the Georgia-Alabama deposits. 



The deposits are in beds or layers, which range in thickness from 

 zero to forty feet, and have a probable average thickness of ten to 

 fifteen feet for the two districts, and two to five feet and more in case 

 of the isolated bodies of ore. 



In chemical composition the Arkansas bauxite varies within wide 

 limits. The granitic type is the purest, and in selected samples con- 

 tains less than 3 per cent, of silica and i per cent, of iron oxide, and 

 corresponds in composition to the formula AI2O33H2O — the tri- 

 hydrate of alumina. In the white bauxitic kaolins the silica ranges as 

 high as 20 to 30 per cent., and in extreme cases the iron oxide reaches 

 50 per cent, in some of the highly ferruginous types of the ore. 



Concerning the origin of the Arkansas bauxite deposits Dr. Hayes 

 says that they are so intimately associated with the igneous rocks of 

 the region that genetic relationship between the two is at once sug- 



