proceedings: geological society 429 



is such as to discourage the idea that accumulation has been accomplished 

 solely through difference in gravity of oil, gas and water inclosed in a 

 porous bed. 



The last meeting for the spring, the 259th, was held May 22, President 

 Stanton presiding. The following informal communications were given: 



E. G. AVoodruff on A bituminous black shale in the Debeque Quad- 

 rangle, Colorado; L. C. Graton on Primary chalcocite ore from British 

 Columbia; and F. L. Hess on A new roscoelite vein near Placerville, 

 Colorado. 



REGULAR PROGRAM 



Structure of the foothills of the Front Range, Central Colorado: G. B. 

 Richardson. The speaker directed attention to a series of strike faults, 

 which heretofore have escaped notice, in the foothills in the Castle Rock 

 quadrangle, Colorado. Stratigraphic evidence of the dislocations is 

 concealed for long distances by outwash deposits, but locally there is 

 proof of profound faulting parallel to the general trend of the Front 

 Range. Six miles south of Palmer Lake, at the base of the mountains 

 just north of Deadmans Creek, a narrow belt of strata of Colorado age 

 standing on edge lies between low-dipping beds of the Fountain formation 

 (Pennsylvanian) on the west and perpendicular Dawson arkose (Eocene) 

 on the east. These relations are due to parallel faults striking north and 

 south. The major portion of Perry Park, 10 miles southeast of Castle 

 Rock, is included between two parallel curved faults. At the eastern 

 end of the park a fault causes the formations from the Fountain to the 

 Colorado inclusive to strike into and abut against the Pierre shale. On 

 the eastern side of the fault the Pierre strikes north and south and is 

 almost vertical; on the western side the strike is northwesterly with 

 dips ranging from 10 to 45 degrees. 



Along the western boundary of the Perry Park block another fault, 

 in general parallel to that just outlined, causes the Fountain formation, 

 dipping 10 to 15 degrees northward, to abut against the Pikes Peak 

 granite in the southern part of the Park, while farther north this fault 

 cuts across the Fountain and forms the boundary between that forma- 

 tion and the Lyons(?) sandstone, the former dipping at an angle of about 

 15 degrees and the latter standing almost vertical. At the northwestern 

 end of Perry Park, the westward continuation of the zone of faulting 

 just described (actual tracing being prevented by a cover of Quaternary 

 wash) apparently causes strata of the Colorado group to lie in close 

 proximity to the Lykins formation (Permian?) . These relations hereto- 

 fore have been considered due to erosional unconformity developed on 

 the so-called Castle "arch." 1 The question was raised whether similar 

 relations at the Golden and Boulder "arches," which in the Denver 

 Monograph are accounted for by unconformities, may not have been 

 caused by a series of longitudinal faults. 



^ee, W. T., Areal Geology of the Castle Rock region, Colorado: American 

 Geologist 29: 96-109. 1902. 



