290 Geologic Story of the Colorado River. — Hill. 
almost vertical, while the Potsdam sandstones and limestones 
are deposited almost horizontally upon their upturned edges, 
showing remarkable disturbance and erosion in middle Cam- 
brian time. Mr. Walcott, probably, saw the Potsdam in 
its regions of least disturbance, for, as shown in our section, 
between Sand Mountain, Llano county, and Marble Falls, 
Burnet county, there was a remarkable crumpling and flexing 
of those strata in common with the Silurian and Carbonifer- 
ous, which accompanied the great igneous intrusions at the 
close of paleozoic times. 
The Silurian history is more obscure ; careful study may 
yet reveal more details of the 1,000 feet of limestone. 
THE DEVONIAN. 
The absence of the Devonian is probable. Dr. B. F. Shu- 
mard intimated the presence of its strata. I made a section at 
Marble Falls to conclusively settle the question and, as final 
authority, sent the faunas to Prof. H. S. Williams for de- 
termination. In my opinion the alleged Devonian is identical- 
ly the Carboniferous limestone of North Texas, which has here 
been intensely metamorphosed by igneous contact. 
The Marble Falls of the Colorado are among its most inter- 
esting and scenic features. Here the river has cut a broad 
valley through the Carboniferous shales ; it^ is dammed back 
by the dip of the underlying limestone formations over whose 
imbricated edges through an extensive caiion the water tumbles 
for a mile or more. In this canon the metamorphism and 
disturbance of the limestone beds bj'^ the adjacent granitic area 
are clearly shown. This section of Marble Falls shows that the 
Devonian sediments were probably not laid down in this 
southwestern region, but if so, they were not later than 
the Corniferous and were eroded by later Carboniferous events. 
The Carboniferous rocks throw much light upon the region 
by showing (a) a complete difference of sedimental se- 
quence from those of the Appalachians, and (b) the prob- 
able absence of the Sub-carboniferous of the Tennessee and 
Missouri sections, or at least their characteristic facies and 
fossils. The presence of Spirophyton, as recorded by Lesquer- 
eux in supposed lower Carboniferous rocks of Arkansas, and of 
Chonetes and other forms, indicates a lower Carboniferous 
position for these limestones, however, while there is a com- 
