1881.1 ^^* [Stevenson, 



further soutli. Limestones are absent and the sandstones show abrupt 

 changes in composition. Direct tracing around the points of bluffs is not 

 always practicable, and identifications of coal beds in different canons are 

 based largely on the intervals found in the typical section obtained on Oak 

 creek. As these inteiwals are by no means constant, some of the identifi- 

 cations may prove to be erroneous. 



The eastern outcrop of the field is well marked by bluffs and it cannot 

 be mistaken, except in the extreme northern part, where erosion has 

 carried away the lower beds and the bluffs contain only the higher 

 members of the series. The sharp hogbacks in which the beds are turned 

 up at angles varying from 40 to 90 degrees, mark the western limit suf- 

 ficiently well, except in the southern part of the field where erosion has 

 beveled off the surface of the mesa. But even there, exposures are found 

 on the larger streams which enable one to follow ovit the limit with but 

 little difficulty. 



Coal mines have been opened at several localities along the eastern side of 

 the trough and branch roads have been constructed by the Atchison, Topeka 

 and Santa Fe and the Denver and Rio Grande railroad companies, lead- 

 ing to mines owned or controlled by those companies. Insignificant open- 

 ings only were seen along the western outcrop, but some pits on the 

 northern side of the field have been worked somewhat extensively to sup- 

 ply Canon City. 



Exposures are infrequent on the northern edge of the field, where a 

 deep covering of gravel conceals all the rocks even to the tops of the hills ; 

 while beyond the sharp ridge bounding the field, a basin has been eroded 

 and rocks are rarely seen in place. The few openings seen afforded no 

 data for determining the position of the beds in which they have been 

 made. A similar difficulty was encountered on the north-eastern side of 

 the basin where the lower beds of the series are burled under the alluvial 

 coating of the plain, and only the higher beds are shown in the bluffs. 

 Some coal blossoms and an extensive opening Avere seen here, but their po- 

 sition was not determined, further than they belong to the upper part of 

 the section. 



Alkali Gap. The stage-road leading from Canon City to Silver Cliff 

 passes through this gap at about 4 miles from Caiion City. The exposures 

 are excellent for a distance of- nearly 400 yards, but as soon as one enters 

 the basin into which the gap opens, all exposures cease. The following 

 section was obtained in this gap, the thicknesses of the several beds being- 

 determined by pacing and calculation from the dip : 



1. Conglomerate 239' 



2. Sandstone 168' 



3. Concealed 40' 



4. Sandstone 128' 



5. Shale 3' 



6. Sandstone 69' 



7. Concealed „ 42' 



PROC. AMER. PHILOS. SOC. XIX. 109. oL. PRINTED KOV. 1, 1881. 



