Geologic Story of the Colorado River .— Hill. 291 
plete unconformity between them and the overlying shales, 
sandstones and conglomerates of the Coal Measures. 
DISTURBANCES CLOSING THE PALEOZOIC. 
Perhaps the two most remarkable features of this sec¬ 
tion are the great igneous disturbances at the close of the pal- 
eozic and Cretaceous respectively. The one at the close of the 
Carboniferous is most beautifully recorded in the southwest 
corner of Burnet county. 
A few miles east of Marble Falls the uppermost paleo¬ 
zoic strata, the Carboniferous shales begin to show much dis¬ 
turbance in the shape of faults, joints and excessive dip. The 
underlying limestones also show this by extensive metamor¬ 
phism as well as by folding until finally a peculiar topographic 
feature known as Shinbone ridge is reached two miles north¬ 
west of the village. This is caused by the lowest or encrini- 
tal limestone strata of the Carboniferous having been thrust up 
almost vertically by the great granite mass which is exposed 
here, and extends nearly ten miles due west to Sand mountain, 
where its western edge is seen to similarly protrude from the 
almost vertical Llano beds, as described by Walcott. This 
great granite outcrop, from which the material was secured 
for the State Capitol, occupies a circular area ten miles in 
diameter, and is of late Carboniferous or post Carboniferous 
age. 1 
This outcrop shows in an unequaled manner the contacts 
of granite and stratified rocks—the beds of the whole paleo¬ 
zoic section showing this in turn—and will prove an admir¬ 
able field for future study. Near the village of Lacy, Burnet 
county, the contact between the granite and the Potsdam 
sandstones can* be followed for miles in a north-east and 
south-west direction, forming waving undulations in the face 
of the steep scarp that marks the western border of the granite 
field. The u Backbone,” another conspicuous topographic 
feature, is an anticline which forms the north-east border of 
the circular area; it is composed of hardest metamorphosed 
limestone of the Carboniferous, and is underlaid by granite. 
This granite core has been uncovered by the erosion of theColor- 
1 The writer believes that Mr. Walcott was justifiable from his obser¬ 
vations to the westward in concluding that all the granite of Burnet 
county was Cambrian, but the evidence here described, which I think he 
did not see, shows it to be of later age.» 
