26 G. K. Gilbert—Post-Glacial Joints. 
before me the desert stretched for a hundred miles—a white, 
glaring plain, interrupted here and there by pecan rocky 
ridges, but for the most part as smooth as a floor almost as 
bare. Through it the Old River Bed sidustloved’ for twenty 
miles, a solitary and abandoned water-course. As I gazed my 
attention was caught by a peculiarity of the farther bank of the 
river-bed, at a point two miles away. ‘The scant rains of to-day 
are washing the silt of the plain from either side into the 
ancient channel, and are at the same time eroding small lateral 
channels. The peculiarity which attracted my attention was 
the parallelism and straightness of a group of these small lat- 
eral channels, and a brief inspection led to the conviction that 
their arrangement was too systematic to be fortuitous. Other 
uties prevented me from making a closer examination, but 
my companion and assistant, Mr. Israel C. Russell, was enabled 
to do so the following day, and he found that the details of the 
drainage were controlled by a compound and extended system 
of joints. The principal series trend almost precisely north and 
south and a subordinate series east and west. They all are 
vertical and straight and (within each series) slcaty "peated 
They are readily traced from top to bottom of the walls of the 
lateral ravines, and not infrequently a wall exhibits a broad, 
flat, sheer face ‘caused by the removal of the clay from one side 
of a plane of jointing. Elsewhere the faces of the bluffs are 
buttressed by square hee rile or nog ei by outstanding 
rectangular columns, the forms o ich have been determined 
by the two systems ‘of seit The main arroyos leading up 
from the river-bed are controlled by the main system of joints, 
but at a short distance back from the bluff there is a tributary 
drainage at right ssi to the primary, and controlled by the 
cross joints. ‘The edge of the desert plain is thus marked out 
in a series of rudely rectangular blocks which may be regarded 
as the incipient stages of the pilasters of the bluff 
The lamination of the clays and marls in which the joints 
occur is traceable across them, showing that there have been 
no faults upon their wae and the absence of faults is quite 
as strongly attested by the perfect continuity of the even sur- 
face of the plain at a little -distande from the river-bed. 
Mr. Russell’s observations showed that Pir were not restric- 
obliterated. For aught that is known to the contrary they may 
exist in the lake Gas beneath the surface of the entire desert. 
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So ae ea 
Pa ee Ee 
