290 CENTRAL MINERAL REGION OF TEXAS. 



upon persistent lithologic characters, viz. : A, the Potsdam Sandstone, or Lin- 

 gula grits; B, the Potsdam ilags; and C, the Potsdam Limestone. 



DIVISION A. THE POTSDAM SANDSTONE. 



The basal rocks of the Katemcy (Potsdam) Series differ from their prede- 

 cessors in a way which, of itself, implies the degradation of the latter to pro- 

 duce them. They are not always as fine-grained as some particular strata of 

 the Riley Series, but they are generally more comminuted, and they ordina- 

 rily have a yellowish-red color or a paler tint than the Riley Beds. These 

 and the higher beds are well exposed in the valley of Katemcy Creek, Mason 

 County, from which outcrops the serial name is taken. Complete sections 

 of this comprehensive set of beds are not usually to be had in a single dis- 

 trict, owing to the later faults which have broken the continuity of the strata 

 in two, and sometimes three, directions. By putting together the detached 

 sections, as well as may be without more detailed paleontologic study, there 

 seems to be a succession of the following character, beginning below: 



1. A red, rather friable sandstone, often containing Lingulx, and some- 

 times more or less irregularly charged with ferruginous segregations, chiefly 

 Limonite. Thickness variable, say 50 to 100 feet. 



2. A white sandstone, similar in texture to the foregoing; thickness, 10 

 feet to 20 feet. 



3. A very friable greensand, perhaps not always present, but as much as 

 20 feet thick in places. 



DIVISION B. THE POTSDAM FLAGS. 



A set of thin greenish shales and shaly limestones sometimes overlies con- 

 formably the series of grits described above. There may be a doubt of their 

 presence in some sections, but they are well developed on Morgan Creek, 

 Burnet County, where they are fossiliferous. The division is not a very 

 prominent feature of any exposure, but its prior existence can usually be de- 

 tected by the occurrence of a conglomerate at the base of the higher division, 

 which is made up of the waterworn flaky fragments of a similar material. 

 The flags may be allowed a maximum thickness probably of 50 feet. 



DIVISION C. THE POTSDAM LIMESTONE. 



The highest Katemcy division begins, at the base, with the compact green- 

 ish conglomerate above mentioned, of which there is usually 50 feet, with 

 variations in thickness according to the relations of the sections to the oldest 

 shore-lines. There must have been important vacillations of the sea-border 

 during the Katemcy Epoch, but they seem to have been more in the nature 

 of tidal incursions and excursions than local changes of land level. As before 

 remarked, however, there is some evidence of a continuance to this age of 



