ASPEN DISTRICT MAP. 143 
On a hill overlooking Woody Creek the base of the Laramie is brought 
down on the west side of the fault, so as to abut against the Triassic sand- 
stones on the east. From its northern termination, as shown on the map, | 
the fault can be traced along the northeast side of Woody Creek for some 
distance. Its course is marked by a large amount of gypsum, which forms 
a continuous white zone along it. Just below Woody station there is a 
prominent hill jutting out from the Roaring Fork Valley on the northeast 
side, and through this the fault appears to run. Only a hasty examination 
was made, but the Dakota and Niobrara formations on the southwest 
appear to abut against the Triassic red sandstones on the northeast. The 
top of this hill is a dark, vesicular basalt; according to Holmes,' this basalt 
has risen along the fault. No attempt was made to trace the fault farther 
northwest than this point. The southern extension of the fault, also, from 
the point where it leaves the area of the Aspen district map, in the vicinity 
of Little Annie mine, has not been looked for, but it probably grows con- 
tinually less, and if it keeps its normal course runs into granite on both 
sides and is lost. 
RESUME OF STRUCTURE IN THE ASPEN DISTRICT. 
The initial disturbance in the rocks in the Aspen district seems to 
have been a general folding. This folding took place certainly after the 
deposition of the Laramie, and also after the intrusion of diorite and quartz- 
porphyries into the sedimentary beds. It probably followed very close, 
however, upon this igneous intrusion. The deformation seems to have 
been due to a lateral thrust which pushed the sedimentary beds against the 
hard resisting mass of the Sawatch Mountains. The cause of this lateral 
thrust was probably the uplifting of the Elk Mountains to the westward, 
which in turn was due to the intrusion of large masses of molten material 
upward into the sedimentary beds along a line of weakness. A continuation 
of the same force, therefore, which thrust the intrusive rocks into the sedi- 
mentary beds brought about the folding and breaking of these intrusive 
sheets along with the inclosing strata. The disturbance arising from the 
lateral thrust is restricted to a comparatively narrow zone, running parallel 
to the main axis of the Sawatch. The greatest folding occurred along a 
still narrower zone, at some short distance from the granite. Along this 
'Report of the Hayden Survey, 1874, p. 60. 
