114 REPORT UNITED STATES GEOLOGICAL SURVEY. 



ually of a <3onical elevation. Thus, too, with all tbe other forms. Each 

 one will present a characteristic result, incident upon the nature af its 

 surrounding rocks and of the dike-rock itself. 



Sometimes the dikes are found to be in direct connection with the 

 main body of volcanics. In that case they simply represent the filling 

 with volcanic material of fissures formed by volcano-seismic action. 

 What tbe causes may have been that have given rise to the formation 

 of apparently independent fissures must be determined for every indi- 

 vidual instance, as neither the formation in which they occur nor the 

 constitution of the dike-rock affords the slightest clew. 



The importance of these dikes in shaping many of the minor oro- 

 graphic features is not to be underestimated. Comparatively insignifi- 

 cant, if isolated, a group of them can produce a very marked effect. 



METAMORPHOSED ROCKS. 



In contradistinction to metamorphic rocks we may class those that 

 have been changed from their original condition by the direct applica- 

 tion of volcanic heat. 



Kemelted granites I have not observed in Colorado. According to Dr. 

 Loew they occur in New Mexico contiguous to or enclosed in rhyolite. 

 Conglomerates, sandstones, limestones, shales, and volcanic rocks are 

 the ones most frequently subjected to such action. 



Conglomerates are hardened, so that the cement and enclosed bowl- 

 ders appear as if being of "one cast." Commensurate with the petro- 

 graphic character of the enclosures is the change they undergo. Pro- 

 portionate with the heat of the injected lava, and with the conductive 

 power of the rocks penetrated, do we find the extent of the metamor- 

 phosis. 



Sandstones are most frequently changed into quartzites, limestones 

 into marble, and shales are altered into what mineralogists know as por- 

 celain-jasper. The first and last of these are by far harder, far better 

 able to resist attacks of atmospheric agpnts than the original rock. 

 Eeheated volcanic 'rocks are often vitrified, become brittle, and some- 

 times emit a semimetallic sound upon being struck. 



"MONUMENTS." 



As a special feature of much interest the different kinds of " monu- 

 ments" in Colorado may here be mentioned. They may be distinguished 

 according to their method of formation. Four kinds there are, so far as 

 my observations extend. 



Every one is familiar with the picturesque forms of Monument Park. 

 Upon a light-colored (white, grey, or yellowish) pedestal rests the dark 

 protecting cap. The entire monument is composed of sandstone. More 

 readily disintegrating and eroded is the supporting column, hard and 

 firm the stratum which furnishes the caps. Kain, frost, wind, driving 

 sand, and other eroding agents are the artists that produce forms strik- 

 ing for their unique beauty. Throughout the region of Monument Park 

 and the Garden of the Gods they occur, visited and admired by the num- 

 erous travellers passing that well-favored spot. 



Similar to the preceding are those found on Douglas's Creek in 1876. 

 Instead of a sandstone pedestal, hcfwever, we here have shale. 

 Weathered away from the edge of the steep bluff it composes, the shale 

 has been carved into isolated columns. To the superimposed cap of 

 hard sandstone they owe their existence. Reared in a comparatively 



