134 abstracts: geology 



past been referred to the Lewis, Laramie, Puerco, Torrejon, and Wasatch 

 formations. 



The stratified rocks of this area consist of a succession of marine, 

 brackish-water, and fresh-water sediments, which now occur as sand- 

 stone, slate, coal, and conglomerate, in almost every gradation and 

 combination possible. The strata throughout the greater part of the 

 area dip from 1° to 3° toward the center of the basin. The Mesaverde 

 and Lewis formations and the Pictured Cliffs sandstone are accepted 

 as described by previous workers in the San Juan Basin. The beds 

 above the Pictured Cliffs sandstone that have been classed by others 

 as Laramie are divided into mappable lithologic units and described 

 as the Fruitland formation, Kirtland shale, and Ojo Alamo sandstone. 

 A bibliography is included. R. W. S. 



GEOLOGY. — -Some manganese mines in Virginia and Maryland. 

 D. F. Hewett. U. S. Geological Survey Bulletin 640-C. Pp. 

 37-71, with 7 figures. 1916. 



Six deposits of manganese ore are described with regard to location, 

 geology, and occurrence and origin of the ore. Maps show the surface 

 relations and underground workings of three mines. Replacement 

 deposits of four types are recognized — deposits (1) in residual clay, 

 (2) in the clay of a fault zone, (3) along pervious zones in schistose 

 rocks, and (4) in sediments that fill an ancient channel. Five of the 

 deposits are on the east side of the Great Valley in western Virginia 

 and one is near Lynchburg in the Piedmont region. 



The conclusion is tentatively reached that all of these deposits, and 

 probably many more in the belt extending from Maryland to Georgia, 

 were formed while the early Tertiary peneplain was being established 

 in this general region. Four of the deposits outcrop on ridges and 

 spurs that have been so isolated since the peneplain was dissected 

 that they have received little surface drainage since that time, but 

 each of the deposits contains much more manganese than could be 

 derived from the decomposition of a mass of rocks having the volume 

 of the residual clays in which the deposits occur. The clays and 

 manganese minerals have been mined from 200 to 260 feet below the 

 surface and from 170 to 220 feet below water level. In two mines the 

 zone of residual clay and associated manganese minerals extends 105 

 mil 130 feet, respectively, below the level of the nearest streams, 

 which flow in channels cut in bedrock. It therefore appears that the 

 sedimentary rocks of the region may undergo thorough decomposition 



