MAPPING OF THE CRYSTALLINE SCHISTS 873 



ing has been accomplished in a not too early geological time, 

 orographic blocks may cause a distinct offsetting of elevations 

 upon the general surface and display an arrangement in zigzags 

 or en Echelon, which it is difficult to explain upon any other basis 

 than that of normal faulting. Fig. 1 shows by the outline of 

 the wooded area the serrated border of schist outcrops due to 

 this cause. In Fig. 2 are exhibited four distinct orographic 

 blocks of limestone arranged en ichelon and owing their position 

 above the general level to a skeleton work of silica which has 

 fitted them to withstand the gnawing effect of erosion. 



4. Dikes. — It has long been recognized that dikes of igneous 

 or aqueo-igneous rock, and veins whose walls are plane sur- 

 faces, represent the positions of either joint or fault fissures 

 which have been subsequently filled. Especially in the Harz 

 mountains of northern Germany has this method of finding the 

 position of old planes of dislocation been successfully employed. 



5 . Abrupt changes of strike and dip not indicated in folds. — An 

 essential characteristic of every folded area is that gradational 

 values can be found to correspond to all important changes of 

 strike or dip, owing to the fact that folds appear in curves in 

 nearly all of their sections (with undulating crests and trough 

 lines in all sections, otherwise in all sections except those paral- 

 lel to crest and trough lines). With excessively sharp pitching 

 folds, changes of strike and dip by small amounts may be as 

 abrupt as though caused by faults, but search will generally 

 reveal at the apex, in the one case the core of the fold, and in 

 the other the evidences of rupture, although it is quite possible 

 that both will be found together. 



6. Discovery of fault breccia. — Reibungs or fault breccia is 

 very generally found where fault planes of considerable throw 

 are actually uncovered. It is also often found in other places 

 and especially in pits and shafts where access to the wall rock 

 is not possible. Even in these cases, however, it furnishes 

 valuable evidence that faulting has occurred. The formations 

 which are in contact along the fault plane are likely to be indi- 

 cated by the angular fragments of the breccia, which are usually 

 cemented together by calcareous or siliceous material. 



