298 Geologic Story of the Colorado River. — Hill. 
is that of a great monoclinal fold faulted at the axis of the 
great curvature. The faulting occurs in and immediately west 
of Austin, Mt. Bonnell being in the line of upthrust, and the 
numerous north and south turns of the river (see figure) being 
in the line of the faults. This fault line extends south-west to 
the Rio Grande and explains several peculiar phenomena. 
Among these is a remarkable line of springs whose outburst 
causes the beautiful streams at San Marcos, New Braunfels, 
San Antonio and other places. The volume of the Colorado 
at Austin is augmented greatly from these sources. The 
springs are natural artesian wells whose outlets to the surface 
are furnished by the fault lines. The occurence of gold in 
these deep marine Caen-like limestones is also explained by the 
faults ; the gold in aqueous solution has been infiltered from be- 
low but, owing to the porosity of the country rock, is dissemi- 
nated in "blanket leads" instead of concentrated as it would be 
if the containing walls were impervious. 
There is also evidence of extinct warm water springs. Some 
of the disturbances in the lower Cretaceous are of the same 
age as those in the upper, but many of them are much older 
as seen in the differential elevation. 
The upper Cretaceous shows a disturbance of so decided a 
nature that its interpretation is not difficult — This is a great 
basaltic outburst seven miles south-east of Austin. This 
igneous area, as seen from the windows of the Texas univers- 
ity is a double dome resembling the two humps of a camel ; 
on closer examination it is found to consist of a circular body 
of black columnar basalt about one mile in diameter, protrud- 
ing through the chalk, the contacting edges of which are con- 
verted into crystalline marble. Although this is the chief out- 
crop of this igneous rock in Travis county (a score or more 
have been reported along a line westward to the Rio Grande 
and eastward in Bastrop county in the vicinity of Yegua 
Knobs) the rocks in many places show its near approach to 
the surface and, I am inclined to believe, the whole region is 
underlaid by a system of laccolites like those in the Henry 
mountains of Utah described by Gilbert. 
From the serious displacement of the Tertiary strata at 
Yegua Knobs and elsewhere it is probable that the igneous 
protrusions are of early Tertiary age. 
It is impossible here to dwell upon the numerous economic 
