4 BULLETIN 54, U. S, DEPARTMENT OF AGRICULTURE. 
In Paleozoic and Pre-Cambrian time the area which is now the 
Great Basin was alternately above and below the sea, finally attain- 
ing in late Carboniferous time an emergence which was to be perma- 
nent. Its Triassic and Jurassic history is recorded only in fragments. 
Apparently it was largely and more or less continuously above the sea 
and was probably eroded to a low and mature relief. With the end of 
the Jurassic came the birth of the Sierra Nevada and with it the move- 
ments by which the basin was first outlined. The forces and the yield- 
ing of which the nascent Sierra were the expression did not spend 
themselves in this alone, but extended far to the east. At first by fold- 
ing, later by profound and complex faulting, the former region of 
inconspicuous relief was broken into a series of troughs and ranges 
limited on the east by the westward-facing scarp of the Wasatch, as 
on the west by the Sierra. The more prominent lines of fracture 
being north and south, and the accompanying crustal displaceme | 
mainly by monoclinal tilting, there originated the series of erates | 
south trough valleys and of parallel, monoclinal ranges so charactet- 
istic of the Great Basin. 
Extensive faulting is likely to be pictured as cataclysmic, and one 
is tempted to think of the Great Basin as breaking in a day, like 
dropped platter, from its original unity into the hundreds of structur 
fragments that now compose it. Thisisradically wrong. The presen 
structure of the basin has grown very gradually. The movement initi 
ated at the close of the Jurassic has continued ever since and is sti 
in progress. So slow, indeed, has been the development of the relie 
that many streams have been able to maintain what seem to be thei 
Jurassic channels and have cut the rising ranges as fast as they arose 
This did not always happen, and sometimes the streams were turned 
It would seem that different displacements were of different ages and 
have grown with differing rapidities. | 
Neither must it be imagined that the structure is completely simple 
and regular. The general parallelism of valleys and ranges is quite 
unmistakable, but details are much more complex. Ranges sink and 
bend and merge with other ranges; valleys join to other valleys and 
are cut by transverse uplifts; all to make a structure of extreme com- 
plexity, but through which the original simplicity may still be dis- 
cerned. , 
It is impossible to say just when in this slow structural develop- 
ment the region became a “‘basin;”’ probably not for a long time after 
the structure had begun to take form. The whole of the Cretaceous 
and the early part of the Tertiary seems to have been a period of open 
seaward drainage and energetic erosion—an erosion which has severely 
modified many of the ranges. In the early Tertiary this erosional 
period was closed (though not necessarily with causal relation) by a 
period of intense and long-continued vulcanism which is only now 
