FAULT-PLANES IN DAKOTA FIRE-CLAY BEDS 583 



The Section then took a recess. 



At 2 o'clock p m the Section was called to order, Professor Hilgard in 

 the chair. 



The following papers were then read and discussed : 



FAULT-PLANES IN THE DAKOTA FIRE-CLAY BEDS AT GOLDEN, COLORADO 



BY HORACE B. PATTON 



[Abstract] 



Since the field work was completed preparatory to the publication of United 

 States Geological Survey Monograph, number 27, entitled " Geology of the Denver 

 Basin in Colorado," many interesting openings have been made in the Dakota 

 formations in connection with the mining of the fire-clay that characterizes this 

 formation in the vicinity of Golden. Extremely interesting faulting has thus been 

 brought to light. Three distinct kinds of faults are to be seen in the present clay 

 mines. The object of this paper, however, is to call attention to one particular 

 fault, as shown by actual mining operations. The fire-clay bed stands nearly 

 vertical and crops out near the crest of a pronounced hogback. After working out 

 the clay from the surface a tunnel was run from the base of the hogback for the 

 purpose of cutting the fire-clay bed at a lower level ; but the crosscut tunnel failed 

 to cut the fire-clay, although it was run far beyond the place where the clay should 

 have been struck. At length, after fruitless searching for the missing fire-clay, 

 the manager sank a winze from the upper workings following the nearly vertical 

 fire clay bed. When about 10 feet above the level of the crosscut tunnel the bed 

 was found to be faulted by a reverse fault in such a way as to produce a barren 

 gap through which the tunnel chanced to be driven. Such practical applications 

 of the laws of geology to mining operations are of special interest to those engaged 

 in teaching geology. The paper will be published later with detailed drawings 

 which could not be prepared at the present time owing to a cave-in at the mine. 



GEOGRAPHIC DEVELOPMENT OF THE BOLIVIAN PLATEA U 



BY W. G. TIGHT 



I, 



A CROSS-SECTION OF THE COAST RANGES OF CALIFORNIA IN THE VICINITY OF 



MOUNT SAINT HELENA 



BY V. C ORMONT* 



The paper will probably appear as a Bulletin of the Department of 

 Geology, University of California. 



The section then adjourned. 



* Presented by A. C Lawson. 



