e:«dlich. j 



VOLCANICS. 195 



speaking of at present, there are a Dumber of isolated ones, but nowhere 

 were the two rocks jnst mentioned met with. It seems that the magma 

 which upon cooling produced our present trachyte, was existing in 

 enortuons quantities and gave rise to the numerous varieties that now 

 reach altogether a thickness of 7,000 to 8,000 feet. Throughout the 

 entire mass the trachytic characteris coustaut, changinglocally, however, 

 on account of reheating, perhaps. As usual, the tuffs and breccia inci- 

 dent to the formation of the trachyte are found. Were it not for the 

 surprising regularity manifested both in a horizontal and vertical direc- 

 tion, the correct recognition and classiiication otthe many varieties under 

 consideration must necessarily be rendered extremely difficult; altogether 

 impossible for the short time that we could spend among tbem. The 

 schedule based upon observation of the various strata at numerous 

 points will give a general idea of the vertical distribution. Single 

 features of the same strajta were fou:id to be of great regularity and 

 materially aided in the identification. 



No. 1 is very readily distinguished by the variegated appearance 

 produced by a succession or change of different colors. Frequently the 

 white, grayish, or yellowish colors predominate, but pink, red, green, 

 and almost black are not wanting. In general appearance the members 

 of this number present the characteristics of a series of "variegated 

 marls." They form steep walls, weathering in small columnar masses, 

 or they show smooth, rounded bluffs, the colors of which are beautifully 

 blended. Should any isolated hard strata be contained in the series, 

 they will show themselves as small monument-shaped projections on 

 the face of the bluff, or weather in such forms that the play of fancy 

 can readily picture them as imitations of animate beings. Water-courses 

 cut deeply into the loosely-cemented material, and aid in forming the 

 picturesque grou])s that may often be observed. On one of the branches 

 of Lake Fork Creek a very curious group of "monuments" was 

 observed in the trachyte of this number, illustrated by the accompany- 

 ing cut. From the high plateau upon which stations 3 and 4 are 

 located, broken fragments of basalt have rolled down the steep hill and 

 found a resting-place on a small grassy slope near the creek. Heavy 

 rain-storms gradually eroded the soft underlying material. This, protected 

 in the vertical direction by the basalt block, assumed a columnar form 

 in course of time, affording a sufficiently large resting-place to the rock 

 that has produced this striking result. At the time of our visit, there 

 were quite a large number of these "monuments" clustered together 

 in a ravine that had been thus washed out. With the progress of erosion, 

 the top of the conical pedestal holding the heavy block must assume 

 a still more conical shape, and the latter will fall. Several of 'these 

 columns were between twenty and thirty feet high. Differing from those 

 in Eastern Colorado, these monuments do not show smooth sides, but a 

 corrugated surface, produced by the constant dripping of rain during 

 storms and other similar reasons. 



This trachyte ^o. 1 reaches an average thickness of about 800 feet 

 when fully developed. It might pro[)erly be termed a tuff'- although at 

 times layers occur in it that would forbid any such appellation if found 

 isolated. Generally the material composing this series of strata js a 

 light feldspathic aggregate loosely cemented. At some points larger 

 fragments are found among the smaller ones, but all show alike the 

 tendency to rapid decomposition. The nuuierous color's that frequently 

 occur are due to oxygen compounds of iron mainly. At some points, 

 where the next stratum above seems to have reached No. 1 in a heated 

 state, the upper members are baked and contain jasper and semi-opal. 



