CONCLUSION 195 



should expect the older fault-planes to show offsets; for if the block, 

 moves on several fault-planes at once, it must move parallel with their 

 several lines of intersection, which must therefore be parallel with each 

 other. Wedge-shaped blocks are sometimes represented in geological sec- 

 tions bounded by intersecting faults with the movement shown by the 

 displacement of the strata, but with no offset shown on either fault. This 

 is an impossible arrangement. 



Not infrequently blocks are represented as displaced and tilted in ways 

 which can not, apparently, be accounted for unless some of them have 

 been plastically deformed ; and there is no evident reasons why these par- 

 ticular blocks should have been singled out for such deformations. These 

 matters are mentioned here in order to call attention to the necessity of 

 considering what displacements are possible with blocks which are nearly 

 rigid, when sections must be drawn from incomplete data. 



Conclusion 



The varieties of complex faulting are very great, and no attempt has 

 been made to treat them all. The method of treatment has been fully 

 set forth and a number of examples given. The method is so simple that 

 it can be mastered in a very short time; and other cases can then be 

 treated without difficulty. The projections show at a glance the com- 

 plete structure. Where folded strata exist their forms must be indicated 

 by their proper contours. The confusion due to the multiplicity of lines 

 can be avoided by using different surfaces, by rubbing out construction 

 lines after they have served their purpose, and by drawing separate dia- 

 grams to represent the structure between successive selected levels where, 

 as in the case of mines, the tunnels increase the complexity of the draw- 

 ings. To persons unfamiliar with geometrical projections, the method 

 may at first seem difficult to use ; but the nature of the problems requires 

 the consideration of space relations, and the method here given is prob- 

 ably the simplest possible method of dealing quantitatively with these 

 relations; and without quantitative methods we can not get quantitative 

 results. 



Sometimes the character of the movement on a fault can be inferred 

 from certain general considerations, such as the prevalence of normal or 

 reversed faulting in the region, from the general nature of the forces 

 which have been active, or from a consideration of the general surface 



XVI — Bull. Geol. Soc. Am., Vol. 20, 1908 



