lxii REPORT OF THE STATE GEOLOGIST. 



also in many cases prove of still greater value in indicating the pres- 

 ence of the bed of magnetite from which they were originally derived. 

 Other similar ores are found showing still further alterations and 

 the addition of water, but while such ores are somewhat abundant, 

 richer ones will probably prevent their being brought into use. 



SILURIAN SYSTEM. 

 The next system, the Silurian, began with shallow water conditions 

 on the west of the Central Mineral District, followed by a subsidence, 

 during which conditions the strata of the Leon and San Saba series 

 were deposited. 



THE LEON SERIES. 



The Leon Series consists of sandy, shaly buff and sometimes yellow 

 dolomite, overlaid by beds of siliceous magnesian limestone. These 

 rocks have two distinct methods of weathering, the lower containing 

 numerous caves, while the upper forms steep bluffs. These are followed 

 by the compact limestone of the Hoover Division. This widespread 

 series of rocks, some of which are better known as Burnet and San 

 Saba marble, lithographic stone, etc., is found both in contact with the 

 other strata just described, and also upon various horizons of the Pots- 

 dam sandstones and limestones. They consist of fine-grained, compact 

 limestones, or white and gray dolomites. At the base of the series they 

 contain a large number of fucoidal remains, and become finer-grained, 

 and thicker and purer as we ascend, until toward the close another ele- 

 vation of the sea bottom gives us fine-grained, gritty dolomites, now of 

 crystalline texture. 



THE SAN SABA SERIES. 



The San Saba Series, which overlie these and form the upper beds of 

 the Silurian System, consist of dolomites and chert, and it is from 

 these that much of the material for later conglomerates and sandy shale 

 is derived. The rocks of the Silurian are therefore limestones and do- 

 lomites, which in many cases have been metamorphosed into marbles 

 or semi-marbles. They are found surrounding the entire Archaean 

 area, and recent investigations point to their possible continuation west- 

 ward (under the cover of the Cretaceous rocks) to the mouth of the 

 Pecos, and it is quite probable that they formed the eastern shore of the 

 interior sea also. They are also found well developed on the eastern 

 flank of the Franklin Mountains, near El Paso. 



