528 PRINCIPLES OF STRATIGRAPHY 



Fault Breccias, 



So far as understood, fault breccias due to thrusting and those 

 due to gravitational readjustment are not essentially different in 

 character, except that the former may frequently be parallel to the 

 bedding planes of stratified rocks in which such thrusting has taken 

 place, while the latter are likely to cross these planes at an angle. 

 Both are, however, secondary features formed within the rock 

 mass, after its deposition if not its consolidation. In these dynamic 

 movements, autorudytes, autoarenytes, and autolutytes result, and 

 they are often accompanied by the formation of slipping surfaces or 

 slickensides. The coarser rocks of this type often simulate hydro- 

 elastics, owing to the fact that during the movement of the rock 

 masses past each other the angles of the fragments were worn off 

 and have become rounded by mutual friction. (Van Hist-^o-.S/g.) 

 In this condition the bed may readily be mistaken for a hydroclastic 

 conglomerate (hydrorudyte). Such beds have been termed pseudo- 

 conglomerates. Typical autorudytes or dynamic breccias consist 

 generally of more or less angular fragments embedded in a matrix 

 of crushed material. In all cases the autorudytes are composed 

 of the material of the enclosing rock with the fragments derived 

 from the most brittle of the beds from which it was formed. Thus 

 an autorudyte formed from interstratified layers of limestone and 

 quartzite will have its fragments mainly composed of the quartzite. 



Since autorudytes so often simulate normal conglomerates, it 

 becomes important to determine the criteria by which the two may 

 be distinguished. For it is obvious that, if the former is mistaken 

 for the latter, it will lead to an entirely erroneous interpretation of 

 the history of that region. This is particularly the case where the 

 autorudyte may be mistaken for a basal conglomerate. Alternating 

 beds of shales or pure lutytes, and fine lutaceous arenytes or gray- 

 wackes, may by deformation be broken up in such a manner that 

 the arenytes yield pebbles, more or less rounded by mutual fric- 

 tion, while the shale flows and fills the spaces between the frag- 

 ments. (Van Hise-30.) 



The pseudo-conglomerate thus produced will have "a slate ma- 

 trix and pebbles of graywacke [argillaceous silicarenyte] which, so 

 far as its own characters are concerned, could not be discriminated 

 by any one from a true conglomerate." (Van Hise-30.) Auto- 

 clastic limestone pebbles of a rounded character are common in the 

 Cambro-Ordovicic rocks of the Taconic region, and in other dis- 

 turbed Palaeozoics in many portions of the world. They are ex- 

 tremely characteristic of pre-Cambric rocks. 



