118 GEOLOGY OF THE DENVER BASIN. 



types of faults mentioned here, but their definitions do not appear to the 

 writer in be sufficiently broad to cover all cases, especially in regard to the 

 omission or repetition of beds along a certain line. 



From the definitions given above it is evident that the "normal" and 

 '•reverse" types of faults bear a supplemental relation to each other. 'There 

 are also certain interesting conditions tinder which these types are liable to be 

 confounded with each other. This may he shown graphically by construct- 

 ing ideal fault sections of the two types, as a anil h in figs. I and .">, PI. XII 1. 



Revolve these sections in the direction of the arrows, bringing them 



into the positions c and </, respectively, or, more generally, revolve each 



section through the several quadrants of a circle, and the conditions giving 

 rise to the confusion of types at once become evident. Indeed, were it 

 not for the arbitrary decision as to what characteristics should serve to 

 distinguish the two varieties of faults, especially those characteristics given 

 in the second part of the definition, which are really to he regarded in the 

 light of tests, it should he an easy matter to assign either of the faults in 

 the above figures to that type with which in reality it had no direct affinity 

 whatever. It is possible, moreover, for either type of fault to have origi- 

 nated from compression or from a sinking of the strata on the downthrow 

 side of the plane. In this event, however, one of the guides to ,i correct 

 solution of connected problems will be found in the law o\' geographical 

 occurrence of the two types, which is, for mountain regions and regions of 

 well-developed folds, the "reverse" fault, resulting from compression; for 

 prairie regions or regions devoid of sharp folds, the normal fault derived 

 from monoclinal fractures and a sinking of the earth's crust. 



A fourth type of fault, in which the plane coincides with the plane of 

 stratification, the movement taking place in either direction along such plane, 

 has not been described here, owing to its not having been observed in the 



Denver held. In its reference it might be regarded as belonging to any one 

 of the foregoing types, the line of fracture taking place along instead of 

 across a stratification plane. 



Analysis of the South Boulder Peaks fault. All analysis of tile structural features 



of this fault refers it to the "reverse" type. If the fault plane, with 

 westerly hade of 60° to 70°, and the planes of stratification to the 



