862 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. 



often called " barns". Such appellations as " natural bridge", " gate", 

 "gateway", "tunnel", and others, designate each a definite class of forms. 

 They are too generally understood to require any discussion as to ety- 

 mology and comparative applicability. Occurrences of this character 

 have always been invested by man with more or less mythical interest, 

 and many a legend is told which stands in the most intimate connection, 

 therewith. 



ISOLATED FOKMS. 



As "isolated forms" we may regard such that vary in their distribu- 

 tion and mode of occurrence from the classes above enumerated. Tak- 

 ing for instance the trachytic conglomerates: we may regard the occur- 

 rence of grotesque forms within that group as a characteristic thereof. 

 It is an essential feature, and one that may readily be employed in 

 determining this recognition. In contradistinction thereto, isolated 

 products of erosion are such that occur only sporadically in geognostic 

 groups or formations where they would not be expected. Some pecu- 

 liar circumstances may combine to produce such results, and in vain 

 may similar forms be looked for at other localities within the same 

 horizons. From the nature of the case, it is evident that the isolated 

 monumental products will occur comparatively rarely, and that they 

 will show a great diversity of composition and shape. Only a few such 

 instances will be mentioned from Colorado as comprising the most 

 prominent representatives of this class. Extensive erosion within cer- 

 tain regions will necessarily result in the formation of objects that would 

 appropriately be placed under this head, but for our purposes it will be 

 entirely sufficient to refer to but a few of them. 



lizard's head. 



The Mount Wilson group is located a short distance west of San 

 Miguel Lake, in west longitude 107° 59' and north latitude 37° 50' 

 Descending by the Bear Creek trail from the divide between Eio Ani. 

 mas and San Miguel drainage, we see before us a steep, downward 

 slope which abruptly terminates in the valley of the last-named river. 

 Two thousand feet below us lies the placid sheet of water which receives 

 its name from the river. Looking beyond it toward the northwest, we 

 see the mountain mass of the Wilson group rising high up in bold relief. 

 An elevation of 14,280 feet is reached by the main peak, the summit 

 being nearly a mile higher than the lake. Prominent in the mountain 

 group we notice a "needle" standing near its eastern edge.* From a 

 distance it appears insignificant, but we can easily determine that it 

 must be of large proportions in order to be seen at all. As we approach 

 closer, we find that a comparatively regular pedestal has been formed, 

 supporting an enormous monolith. Steep slopes lead up to its base, 

 broken often by vertical walls. From this base rises a gigantic rock- 



*Coiui)are Annual Report for 1874, fig. 2, page 207. 



