76 NEW YORK STATE MUSEUM 



fault breccia was encountered so perfectly reliealed that the rock is 

 completely crystalline and does not develop weakness on weather- 

 ing. A second fault of similar behavior developed a very fine shear 

 zone which crosses the northwest corner of lona island and strikes 

 northeast across the west flank of Anthony's Nose. It is so sub- 

 stantial that the rock within the zone, which is essentially an epidote 

 schist, is as resistant as the adjacent unmodified ground. Other 

 similar shear zones have been noted elsewhere and beautiful shear 

 schists of most remarkable structural habit have been obtained from 

 them (see petrographic section), but there is no topographic expres- 

 sion to emphasize the occurrence of the faults of this type and unless 

 one just happens upon them they are overlooked. They all fall, 

 however, into the lines of general structural trend and undoubtedly 

 they belong to some period of Precambrian deformation. (See 

 plate 42 for illustration of the microscopic features of this material.) 



All Cambrian and later faults are probably so imperfectly healed 

 in this region that their occurrence is more readily detected. 

 Although there may be several series, it is doubtful whether more 

 than three can be well established — those belonging to the period of 

 the Taconic folding, those of Appalachian mountain making and 

 those belonging to the Triassic period. Even this much is more 

 than can be differentiated very accurately. 'The first two are especi- 

 ally confused and perhaps in some cases there has been movement 

 along the same line in more than one period. 



It is possible, however, to locate definitely a number of faults 

 which are of large displacement. The Highlands belt itself is an 

 up-thrust block with fault boundaries to the north and south. The 

 fault lines run obliquely to the general trend of the belt itself instead 

 of exactly parallel to it, and it thus happens that cross-faults give 

 an irregular saw-tooth effect to the boundary. Although several 

 have been definitely placed on the map and others are drawn tenta- 

 tively on the basis of structural weaknesses or radical change in 

 formation, it is certain that many more faults actually occur, and that 

 the district is much more complicated than is shown by the accom- 

 panying map. 



The faults of largest determinable displacement are the one at 

 the north margin of the Highlands along Breakneck mountain, and 

 the one which extends from Tompkins cove northeastward along the 

 west margin of the fault block of Peekskill valley. In both cases the 

 displacement must be more than 2000 feet to cut out the formations 



