REVIEWS 663 



The ore deposits of later age are placer deposits along the numerous 

 streams that head back in the central and older rocks of the region. Of 

 all these the Deadwood placer is the richest. 



The great number of admirable diagrams illustrating structure, and 

 also the many excellent photomicrographs of the ores, are among the most 

 valuable features of the report. The petrographic study of ore deposits is 

 rightfully coming to command greater and greater attention from mining 

 men as well as from scientists. 



W. D. S. 



Zinc and Lead Deposits of Northern Arkansas. (Professional Paper 

 No. 24, U. S. Geological Survey, 1904.) By George I. 

 Adams, assisted by A. H. Purdue and E. F, Burchard. With 

 a section on "The Determination and Correlation of the Forma- 

 tions" by E. O. Ulrich, Pp. 113, and 27 plates and maps. 



This paper is a preliminary report rather than a final treatise on the 

 district with which it deals. It represents a further prosecution of the 

 study of the whole Ozark region wherein the same general principles of ore 

 deposition will, doubtless, be found to prevail. It is very timely, as the 

 scientific exploitation of the area cannot be said to have more than begun. 

 The prospect holes greatly outnumber the productive workings, and the 

 mines that are in operation lead a rather spasmodic existence. 



The report deals with an area, comprising Marion county, the northern 

 part of Searcy, the eastern border of Boone, and the northeastern part of 

 Newton county, coming within the Yellville quadrangle, which lies in the 

 much dissected plateau portion of the Ozark region, and is underlain by 

 comparatively undisturbed sedimentaries that dip shghtly southward 

 toward the Boston Mountains. 



These sedimentaries comprise Ordovician, Silurian, Devonian, and 

 Carboniferous strata which have been moderately folded, fractured, and 

 brecciated. 



Ore was reported as occurring in this district by Schoolcraft as early 

 as 181 8. There are two ore horizons, the Yellville dolomite, Ordovician, 

 the oldest formation exposed in the area, and the Boone chert of the Mis- 

 sissippian series. The ores are chiefly zinc blende and galena, but there 

 is also a minor quantity of oxidized ore mined from the upper workings, 

 and some zinc silicate, calamine. 



The general movement of underground water, which was the medium 

 by which the ores were concentrated, was essentially the same as that 



