1877. ] Explorations in Colorado. 75 
From this point the party proceeded westward across the San 
Luis valley and up the Rio Grande to its source, making two 
primary stations on the way, one near the summit district and 
the òther on the Rio Grande Pyramid. From the head of the 
Rio Grande the party crossed the continental divide, striking 
the Animas Park, thence west by trail to Parrott City. 
After making a station on La Plata Peak, the party marched 
northwest across the broken mesa country west of the Dolores, 
making three stations on the route to complete a small piece of 
topography that had been omitted the previous year on account 
of the hostility of the Ute Indians. After making a primary 
station on the highest point of the Abajo Mountains, the party 
turned westward to Lone Cone, where another station was made ; 
thence, crossing the Gunnison and Grand rivers, they proceeded 
to the great volcanic plateau at the head of White River. The 
final station was made between the White and Yampah rivers in 
the northwestern corner of Colorado. During this brief season, 
Mr. Wilson finished about one thousand square miles of topog- 
raphy and made eleven geodetic stations, thus connecting to- 
gether by a system of primary triangles the whole of Southern 
and Western Colorado. 
In company with the triangulation party, Mr. Holmes made a 
hurried trip through Colorado, touching also portions of New 
Mexico and Utah. He was unable to pay much attention to de- 
tailed work, but had an excellent opportunity of taking a general 
view of the two great plain-belts that lie, the one along the east- 
ern, the other along the western base of the Rocky Mountains. 
For nearly two thousand miles of travel he had constantly in 
view the Cretaceous and Tertiary formations, with which are 
involved some of the most interesting geological questions. He 
observed among other things the great persistency of the various 
groups of rocks throughout the east, west, and north, and espe- 
cially in the west, for from Northern New Mexico to Southern 
Wyoming the various members of the Cretaceous formation lie 
in almost unbroken belts, while the Tertiaries are hardly less 
easily followed. 
Between the east and the west there is only one great incon- 
gtuity. Along the eastern base of the mountains the upper Cre- 
taceous rocks, including numbers four and five, are almost wanting, 
consisting at most of a few hundred feet of shales and laminated 
sandstones, Along the western base this group becomes a prom- : 
ment and important topographical as well as geological feature, 
