ENDLicH.] ACIDIC VOLCANIC ERUPTIVES TRACHYTE. 221 



to what may be regarded as an incidental cause, and its accompaniment 

 of trachyte No. 2 is not an invariable occurrence. Dependent upon 

 favorable orographic and hypsometric conditions, it has been deposited 

 in varying masses along the border of the former volcanic peninsula. 

 There is scarcely any doubt that the waters into which the conglomerate 

 was deposited occupied but a restricted area as compared to that 

 covered by immediately preceding formations. After the ejection of 

 the lava which composes the flows of the Eiver group, a peripherical 

 subsidence probably took place, permitting the accumulation of water 

 along the borders of the volcanics. I have not enumerated the con- 

 glomerates as an independent group in the list of volcanics of Colorado. 

 This omission has been made, because I regard its presence and its de- 

 velopment as essentially accidental. Very few extensive groups, indeed, 

 of volcanics occur anywhere without being accompanied, more or less, 

 by tuffs and breccias or conglomerates. 



Structurally the conglomerates present the usual type. At some lo- 

 calities a separation of the coarse from the fine material was observed. 

 Not unfrequently " bands " seem to occur in the conglomeritic walls, 

 often continuing and visible for a long distance. Upon examination, 

 these bands prove to be of different physical composition. Mostly they 

 are composed of more uniform (in size) finely-separated material, which 

 is either cemented together more loosely or more firmly than the sur- 

 rounding masses. 



An examination of the lithological character of the bowlders con- 

 tained in the conglomerates shows that we have before us the larger 

 quantity as resembling the trachytes of No. 2. Higher up, near the 

 upper termination of the breccia, different rocks set in, however. Some 

 of them closely resemble the porphyritic trachytes, others can scarcely 

 be found to differ from types that we find in the younger groups of the 

 trachorheitic series. I am inclined to the opinion that local eruptions 

 took place during that period of time when the main volcanic group 

 was nearly surrounded by w^ater, and that they furnished the material 

 for this rare species of conglomerate. At a number of localities evi- 

 dence remains of the deposition of the breccia into water. Dr. Peale 

 has noticed* its " stratified appearance," and along the western border 

 of the mountains such evidence was frequently observed.t Near Station 

 36 of 1875 a layer, irregular in its vertical dimensions, of pumice was 

 noticed near the top of the conglomerates. Upon investigation it proved 

 to have been, originally, obsidian, which flowed into the water receiving 

 the breccia. 



Characteristics not only of physical composition, but also of physical 

 appearance, are exhibited by the conglomerates. As a rule, they are soft, 

 easily eroded, readily cut by flowing water. This, in connection with 

 the irregular distribution of bowlders and softer portions, is productive 

 of very marked results. "Water, wind, and frost carve the most fantas- 

 tic figures out of the yielding material. Steep, frequently vertical, bluffs 

 are ornamented by many varieties of forms. Caves and arches are 

 formed, the former not seldom affording shelter to some of the wild ani- 

 mals infesting the region. In case the breccia be firmly cemented, these 

 detail-features will, in a great measure, be lost. Steep walls, either as 

 bluffs or in caiion?, with but scattering monumental ornamentation, 

 characterize the harder varieties of the rock. Throughout the volcanic 

 area, wherever the conglomerates were observed, these characteristics 

 were found to hold good. By their aid this group can readily be dis- 



* Ecip. U. S. Geol. Surv., 1873, p. 94, 



+ Compare Rep. U. S. Geol. Surv., 1875, p. 213. 



