100 REPORT UNITED STATES GEOLOGICAL SURVEY. 



in widely-extended flows, but built up a small range or group of mount- 

 ains. Physically the rocks of this series resemble the unstratified crys- 

 talline aggregates more than the volcanics of the first group. In the 

 paste there is contained a by far larger quantity of segregated, crystallized 

 minerals, which causes the lithological character of the rock to resemble 

 that of fine-grained granite more nearly than that of a trachyte. Con- 

 sequently we find, too, that so far as erosion and the influence of atmos- 

 pheric agents are concerned, the volcanics of this group closely resemble 

 granite. By their isolated position, however, and in their relations to 

 the topography and orography of the neighboring regions, they are 

 readily distinguished. 



Volcanic eruptions. — Volcanic eruptions are so termed in contradis- 

 tinction to the massive outflows. Within Colorado none were positively 

 determined, and I very much doubt whether there ever will be. The 

 existence or indication of a "crater" built up by the outflowing lava is 

 essential. Cone-shaped mountains are thereby produced, from the cen- 

 tre of which the liquid lava is poured through the main crater and lat- 

 eral fissures, over a sometimes considerable extent of country. Many 

 of the eruptions observed in Colorado approach these in general char- 

 acter, but are not sufi(icie:!tly typical to receive even the name. They 

 belong to a different class throughout, and cannot justly be compared to 

 the former. 



Dilces and dike-systems. — Among the volcanic formations these are 

 some of the most interesting, both as regards their distribution and the 

 features they exhibit upon being exposed to eroding influences. Dikes 

 show themselves on the surface generally as more or less regular walls, 

 or form narrow, sharp ridges. One occurrence that must be classed 

 among them is that where the material composing them has reached the 

 surface, and, gradually overflowing, has formed cofiQn-shaped or cone- 

 shaped hills and mountains. 



Simple dikes penetrate mainly sedimentary beds, and, upon the grad- 

 ual removal, by erosion and transportation, of the contiguous beds, 

 they stand out prominently. Disintegration of the hard volcanic rock 

 composing the dike, widening of originally narrow fissures, or removal 

 of certain portions by water, produce breaks in these walls. In that 

 case they may frequently be rei)resented by isolated, columnar masses, 

 the general distribution of which indicates the course of the unbroken 

 dike lower down. Orographically they impart to a region where they 

 are found a j)eculiar character. Although, as a rule, they stand out 

 prominently, not disturbing the general level of the adjoining beds, they 

 sometimes give rise to the formation of narrow ridges and hogback- 

 shaped hills. This is accomplished either by their protecting from ero- 

 sion these beds, or by having rendered them more resisting through par- 

 tial or complete metamorphosis. Isolated knolls and buttes are fre- 

 quently formed by the remnants of once continuous dikes, of which 

 l)ortions have been worn away to the present level of the region in which 

 they occur. 



Dike-systems have by far more influence in determining surface features 

 than simple dikes, and are not unirequently the original cause of pro- 

 ducing prominent hills and mountains. It will generally be observed 

 that a dike system has either one centrjil starting-point, or one line from 

 which the single dikes emanate. If these are sufficiently near together, 

 or have hardened, hy alteration, the media through which they have 

 passed, they will aftbrd ample protection from erosion to the strata they 

 have traversed. Thus every dike will form the crest of a narrow ridge 

 leading toward the central point or line of outflow. In case the dikes 



