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C. D. Walcott—Paleozoie Rocks of Central Texas. 431 
Art. LL—wNote on Paleozoic Rocks of Central Texas ; by CHas. 
. WALcorT, of the United States Geological Survey. 
THE writer had the opportunity the past season to make a 
hurried reconnoissance of a portion of the Paleozoic area of 
Central Texas: the chief object in view being the study of the 
Cambrian section and the collecting of fossils from the Texas 
Potsdam horizon. 
At all localities where the base of the Potsdam was observed, 
it rests, wnconformably, on a great formation that is stratigraphic- 
ally the equivalent of Powell’s Grand Cafion series (Grand 
Cafion and Chuar groups).* In the Grand Cafion of the Colo- 
rado the latter are overlaid by the Tonto group, a series of 
rocks, in both lithologie and paleontologic characters, singu- 
larly like the Texas Potsdam group. 
For this series of Pre-Potsdam strata the local name of Llano 
group is proposed from the best exposures of the group occur: 
ring in the county of Llano. Outcrops also occur in Burnet, 
Mason, San Saba, Blanco and Gillespie counties. 
he finest exposure seen by the writer, in direct contact 
with the base of the Texas Potsdam group, is along the west- 
ern base of Packsaddle mountain, in Llano county. Here the 
massive reddish colored sandstones of the Potsdam strike north 
and south with a slight dip to the eastward, and rest on alterna- 
ting beds of shale,-sandy shales, sandstone, limestone and 
schists that strike east and west, dipping south 15° to 40°. The 
strata exhibit but little evidence of metamorphism, being indu- 
rated but little more than the beds of the overlying Potsdam 
and Carboniferous. The section shows the Llano and Potsdam 
groups unaffected by changes subsequent to the consolidation 
of the Potsdam sediments. 
Across the valley of Honey creek, four miles west of Pack- 
saddle mountain, the strata of the Llano group have been more 
metamorphosed, plicated, and broken by intrusive dykes of 
granite. This is along the eastern base of a ridge of Potsdam 
Silurian and Carboniferous rocks that strike eastward with a 
dip that increases from 10° at the north end of the ridge to 40° 
at the south end. The movement producing this position, 
as compared with the Potsdam beds of Packsaddle moun- 
y the result of extrusions of granite at or near the close of the 
* This Journal, vol. xxvi, p. 437, 1883. 
