194 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 10, 



direction, as exposed for 50 or 60 yards, is E.N.E., being nearly par- 

 allel to the dislocations at Deptford and Greenwicli*. It is attended 

 by a slight anticlinal line of flexure, indicative of the lateral pressure 

 which was likely to give rise to fractures of this peculiar character, 

 and the eifects of which are seen in the contorted strata at New 

 Cross f . 



The second line of fracture, which dies away both upwards and 

 downwards, would indicate that the strata sustained a wrench, to 

 which they yielded without suffering complete fracture ; and possibly 

 the main fissure may not be continued to any great depth. 



The subjoined diagram (fig. 2) illustrates the action of the force 

 which may have produced this dislocation. 



Fig. 2. — Diagram to illustrate the Mode of Formation of the Reversed 

 Fault at Lewisham. 



Supposing P to be a centre of maximum vertical pressure, pro- 

 ducing a fault AB, a lateral pressure Q would result, causing a wrench 

 or fracture along a line BC. The same result would follow, if the 

 pressure at P caused only an anticlinal elevation without fracture 

 atAB. 



This theoretical diagram very nearly resembles the actual phseno- 

 mena of a section taken at right angles to the line of dislocation (see 

 section, fig. 3). 



The great obliquity of this fault, viz. 30°, has induced me to in- 

 vestigate a formula for the apparent deviation of a line of fault (as 

 laid down on a map) produced by the undulations of the surface. 



If a, /3 be the inclinations of a fault and of the surface respectively, 

 and y the angle between the direction of the fault and a horizontal 

 line on the surface, then the angle of deviation is given by the 

 formula 



tan (^ 



sm y 



tan a . cot /3 — cos y 



Hence also the inclination of a fault may be determined by observa- 

 tion of the other angles. 



This formula may equally well be applied to mapping the outcrop 

 of strata whose dip is known. 



Applying this to the subject of this notice, the fall of the ground 



* Quart. Journ. Geol. Soc. vol. vi. p. 447, 

 t Loc. cit. p. 448, fig. 6. 



C.5. 



