ENDLICH ON EROSION IN COLORADO. 863 



column, 290 feet in height, while its diameter amounts to from GO to 80 

 feet. Its isolated position permits it to be seen for a long distance, and 

 its elevation — 13,100 feet above sea-level at the summit of shaft — ren- 

 ders it an excellent landmark for all the lower country adjoining. 



Both the rocks composing the Wilson group and those which the 

 monolith — Lizard's Head — exhibits, are of volcanic origin. In former 

 geological periods enormous masses of sedimentary and volcanic ma- 

 terial have been eroded and transported from that region. It seems 

 possible that a former connection existed between the ridge now sup- 

 porting Lizard's Head and the main volcanic group farther east. No 

 surface connection exists at present, however. All that remains in the 

 immediate vicinity of them is the huge monolith. During the period of 

 the great erosion, valleys were cut into the rocks and ridges were grad- 

 ually carved away so as to become narrower and shorter. Probably the 

 disturbances produced by eruptions of volcanic material, and, more par- 

 ticularly, the phenomena accompanying them, rendered the rocks of that 

 region less capable of resisting such powerful agents of demolition as 

 were then employed. It may be observed that the trachytes composing 

 Lizard's Head show a certain development of columnar structure. This 

 structure is almost invariably accompanied by basal fracture-planes. 

 By this means, erosion will be enabled to attack such portions more 

 successfully. A process of undermining will result in the falling of 

 overhanging portions. Owing to the columnar arrangement of the 

 integral parts composing a hill or bluff, the faces produced by such fall- 

 ing will be quite or nearly vertical. In this manner, fluviatile erosion 

 can produce, from such material, a type of form which is represented by 

 Lizard's Head. Had the erosion continued on at the same level, the 

 entire mass must have succumbed. Increasing width and depth of the 

 excavated valleys, however, caused the waters to sink. Thereby the 

 same species of erosion was produced along the sides of that portion 

 which now forms the " pedestal", but the column remained intact. This 

 appears to be the only way of accounting for the existence of Lizard's 

 Head. It is not a dike or intruded volcanic product, subsequent to the 

 main eruptions, but a portion of the regular flows, large masses of which 

 are still preserved not far distant. 



Similar in shape are the forms resulting from a partial breaking-down 

 of mural products of erosion. Their arrangement, however, and the 

 character of the rocks composing them, will admit of their ready identi- 

 fication. 



Another important group of isolated forms of erosion comprises such 

 that are produced by local inclusions of essentially foreign material. 

 Concretions may be contained quite frequently in shales and sandstones. 

 Those to which we have special reference here are harder, resisting 

 erosion and disintegration more effectually than the rocks containing 

 them. Forms similar to those of the monuments may be produced by 

 a gradual wearing-away of the portions adjacent to concretions. Among 



