488 



T. Mellard Eeade — Normal FatilUng. 



have been drawn from actual mining exploration and measurement. 

 The greatest throw was 3^ inches measured on the hade of the fault. 

 Almost every feature of Normal Faulting was reproduced in miniature 

 by this natural diagram, among which are the dropping of the 

 v/edge-shaped blocks (as at e, Fig. 1) ; the universal hade to the 

 downthrow ; the occasional curve of the beds (bands) downwards 

 on one side and upwards on the other as at///, the result of slipping 

 and the tight fitting together of all the parts or wedges into which 

 the strata have been divided by shearing. 



Fig. 1 . — Section of faulted laminated silty clay as exposed in the Cliffs of Glacial 

 Drift at Nevin, Carnarvonshire. Scale ^ inch to a foot (a portion only.) 



a. Banded laminated silty clay (unf aulted) . 



b. Cavity, the underside at x studded with hardened guttse of sand,— the cavity 



in part heing filled with sand. 



c. Similar handed clay to a, hut faulted as shown. 



d. Talus of sand below this. 



It is evident that the faulting of these banded beds is due to their 

 contraction and loss of volume, either by actual drying or by the 

 draining away of their water. Between the overlying unfaulted 

 bed (a) and the faulted bed (c) there was a cavity at (b) six inches 

 wide at the widest part. On the bottom or underside of the un- 

 faulted bed at X were " guttas " or drops of hardened sand, the under- 

 side looking as if studded with petrified raindrops. Probably this 

 cavity was formerly a thin bed of sand which allowed the surface 

 water percolating through the upper bed to drain away without 

 affecting the lower bed. It is not easy to trace the exact modus 

 operandi of Nature in this case, as observation was difficult from 

 the insecure foothold on the sand talus at d. 



An examination of the constitution of this laminated silt by 

 breaking up six ounces of it in water and riddling it showed that 

 a few grains of sand only were caught in a mesh of -A- of an inch, 

 and 10 grains by weight of sand in a mesh xo^o- of an inch. All 

 the rest passed through the xo^o^ inch mesh and f oz. and 10 grs. 

 were caught by precipitation. 



The portion that flowed away without precipitation must have 

 consisted largely of very small granules, probably of quartz, for the 



