HA YD EH SUKVfflf OF THE TEEBITGEIBS FOB 1876 
Work of W. K. Holmes* 
!T In company with the triangalation party, Mr. Holmes made a 
hurried trip through Colorado, touching also portions of Hew Mexico 
and Utah* He was unable to pay much attention to detailed work, but 
had an excellent opportunity of taking a general view of the two 
great plain belts that lie, the one along the east, the other along 
th© west base of the Becky Mountains. For nearly two thousand miles 
travel he had constantly in view the Cretaceous and Tertiary forma¬ 
tions, among which are involved some of the most interesting geologi¬ 
cal questions. He observed, among other things, the great persist¬ 
ency of the various groups of rocks throughout the east, west, and 
north, and especially in the west; that from northern lew‘Mexico to 
Southwest Wyoming the various members of the Cretaceous 11© in al¬ 
most unbroken belts. 
Between the east and the west there is only one great incongru¬ 
ity. Along the east base of the mountains the tipper Cretaceous 
rocks. Including Hos. 4 and 5, are almost wanting, consisting at 
most of a few hundred feet of shales and laminated sandstones. Along 
the west base this group becomes a prominent and important topograph¬ 
ical as well as geological feature. In the southwest, where it forms 
the ’Mesa Verde* and the cap of the Dolores Plateau, it comprises up¬ 
ward of two thousand feet of coal-bearing strata, chiefly sandstone, 
while in the north it reaches a thickness of 3,500 feet, and forms 
the gigantic "hog-back” of the Grand Elver Valley* 
While in the southwest he visited the Sierra Abajo, a small 
group of mountains, which lie in Eastern Utah, and found, as he had 
previously surmised, that the structure was identical with that of the 
four other isolated groups that lie in the same region* A mass of 
trachyte has been forced up through fissures in the sedimentary 
rocks, and now rests chiefly upon the sandstones and shales of the 
lower Cretaceous. There is a considerable amount of arching of the 
sedimentary rocks, caused probably by the intrusion of wedge-like 
sheets of trachyte, while the broken edges of the beds are frequently, 
but abruptly, pressed up, as if by the upward or lateral pressure of 
the rising mass. He was able to make many additional observations 
on the geology of the San Juan region, and secured much valuable 
material for the coloring of the final map. 
Ee states that the northern limit of ancient cliff-builders in 
Colorado and Eastern Utah is hardly above latitude 37° 45 ! .” 
(Page X?) 
Holmes’ report on the Geology of th© Sierra Abajo and West San 
Miguel Mountains occupies pages 189-196 with illustrations. 
