80 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



sea. As no coarse sediment is present, shallow, quiet Carboniferous seas must be inferred. 

 The same type of breakage may locally be observed in stream channels and must not be confused 

 with the basal conglomerate. Beds including the Mississippian (lower Carboniferous), Devonian, 

 SUurian, and later Ordovician are therefore absent. 



As the Ordovician rocks lie immediately above the Cambrian, they occur at the crest of 

 the Paleozoic scarp (unless faulting has intervened) and form the greater part of the Paleozoic 

 surface in Llano and Burnet counties. 



Complete sections of the EUenburger limestone are not easy to obtain. The general mas- 

 siveness of the beds and the gentle folds and faults combine to prevent continuous record. 

 Thicknesses up to 600 feet may be observed in the bluffs of the Colorado between Tanyard 

 Crossing and Deer Creek. It is probable that the formation is 1,000 feet or more thick. 



The following section represents the upper portion measured near the mouth of Flat Rock 

 Creek, Burnet County. 



Section measured in the upper part of the EUenburger limestone near the mouth of Flat 



Rock Creelc. 



Fe3t. 



Carboniferous 50 



Grayish crystalline limestone with much chert 15 



Crystalline limestone, brown and gray, sugar-grained texture, with light-colored chert; coarsely 



crystalline near top, greenish stains in calcite crystals ^ 25 



Massive, hard, smooth-textured beds at base, grading up into brown crystalline and coarse gray 



limestone ; some chert 22 



Alternating sugary and smooth-grained beds; at top a bed of cherty limestone, weathering honey- 

 comb fashion 11 



Alternating sugary and smooth-grained beds; but little chert 11 



Brown, sugary banded limestone with some chert 5 



Smooth light-gray limestone with whitish chert, weathering bluish gray; smooth rounded pieces 



simulating waterworn bowlders on surface 50 



Light-gray smooth limestone, conchoidal fracture; contains some chert which weathers out into 



rough surface; alternating thin and massive 22 



Brown, sugar-grained limestone containing layer of white chert 2 



Irregularly bedded dark and light brown, sugar-grained limestone; tesselated weathering in cliff. 11 



Brown and gray crystalline limestone; lowest portion contains some chert 3 J 



Rough-weathering, somewhat concretionary limestone, sugar-grained, brown and mottled pink. . 2 



Massive beds of brownish-gray smooth limestone, irregular fracture 4 . 



Gray crystalline limestone, massive bedded, sugar-grained, mixed with smooth noncrystalline 



variety 5J 



Brown and light-colored fine sugar-grained crystalline limestone, mostly thin and irregularly 



bedded 17 



Local bowlder bed resembling conglomerate, sandy material IJ 



Brown and light-colored fine sugar-grained crystalline limestone, mostly thin and irregularly 



bedded but makes jagged cliff 27^ 



Bottom at level of Colorado River. 



Certain characteristics of the EUenburger limestone are pronounced. The bedding as a 

 rule is iU. defined, preventing correlation by lithologic units. Repetitions of coarse or fine 

 grained phases of the limestone occur in alternating succession, but the position of individual 

 beds, beca,use of their great similarity, was not determined. An abundance of white and yellow 

 chert characterizes the series, though some layers are free from this material. Where relief is 

 considerable and dissection of the plateau has taken place, the surface of the formation is 

 exceedingly rough, and in general the formation may be recognized by this irregularity of 

 surface. This condition does not apply to the high rolling grass lands to the north. In certain 

 layers the chert has evidently been dissolved and replaced by crystalline quartz filling irregular 

 cavities. 



