THE BAUXITE DEPOSITS OF ARKANSAS 279 



had been in contact with the hot syenites the aluminous mate- 

 rials were se;^gregated as pisolite and sank near where they were 

 formed. Thus if the process is one of decomposition it is 

 decomposition in the presence of and due to high temperature. 



The irregular forms of the deposits, their variable thicknesses 

 and characters, and, in fact everything known about them, is in 

 keeping with this theory of their origin. Stress has been laid 

 by Meunier upon the fact that so many bauxites rest upon^ or 

 are associated with limestones ; and this fact was used by him 

 to explain the precipitation of the material from chlorides of 

 aluminium in hot sea water. But, as Auge has pointed out, lime- 

 stones are not always present,^ and there are certainly none 

 associated with the Arkansas deposits. 



Forms of the deposits. — From what has been said it is evident 

 that the bauxite deposits must be very irregular in form. 

 Although associated with marine sediments of wide and regular 

 distribution, the bauxite deposits are local, irregular and of 

 uncertain extent, for the influences that produced them were 

 local, and the rocks (syenites) with which they are associated 

 are in all probability concealed in many places. The figure 

 given under the head of "origin" on page 275 will give as 

 definite an idea as can be had of the forms of the deposits with- 

 out prospecting. If, at any time, it should become necessary to 

 prospect for other deposits, the prospecting should be confined 

 to the region in which bauxite is now known and to the soft 

 Tertiary beds. It does not now seem advisable to look for this 

 mineral above an elevation of 350 feet or below 250 feet above 

 mean tide. 



MetJiod of mining. — Stripping and quarrying in open cuts is 

 the method to be used in nearly all the deposits known. Of 

 course when the cover becomes too thick it cannot be removed 

 economicall}^, and such deposits will either have to be aban- 

 doned or the mining will have to be done by drifts. Persons 

 experienced in the driving of tunnels and mine timbering should 



' Comptes Rendus, 1S83, XCVI, 1737-1739. 

 » Bull. Soc. Gdol. de France, 1888, XVI, 346. 



