FIELD GEOLOGY. 209 



On the N. side of the river the limestone does not 

 come within the area ; the line of the sand is run as 

 before, but after tracing it for about half its length we 

 meet with a break in it similar in every respect to that 

 on the other side of the river. It seems also to be in 

 the same direction or thereabouts ; indeed it fits in with 

 the provisional line of fault when the latter is suffi- 

 ciently extended there can now be no doubt of the 

 existence of a dislocation. This fact is further corrobo- 

 rated by the coal-crops on the K, which behave exactly 

 in a similar manner to those on the S., except that one 

 portion of the line has been thrown back somewhat 

 towards its former position. When the outcrop has 

 been carefully mapped, it is found that this second 

 throw can be explained in one way only that is, by 

 another fault, nearly at right angles to the first, passing 

 through the broken ends of the coal-seam, as shown in 

 the drawing, fig. 21. 



Downthrow. The amount of downthrow at any part 

 of the fault can be easily calculated from the known dip 

 .and the lateral shift of the beds where crossed by the 

 line of section, at * fig. 22, it is 107 feet the following 

 being one method of its calculation. From * (in this case 

 on the end of the unshifted part of the coal-seam) to the 

 nearest part of the shifted portion (i.e. in the direction 

 of the rise) the distance is 200 yards the fall of the 

 ground between, as ascertained by aneroid or otherwise, 

 being 20 feet. By Table, Part II. p. 50, the depth below 

 the surface attained by the coal-seam, with a dip of 8 

 in 200 yards (-600 feet), is 87 feet, which added to 

 the 20 feet fall=107 feet, the amount of downthrow at 

 the point *. The result would be arrived at, if the 



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