DR. 0. WARD ON SLICKENSIDES OR ROCK-STRIATIONS. 113 



8. On Slickensides or Rock- striations, particularly those of the 

 Chalk. By Dr. Ogier Ward. (Read March 25, 1874.) 



(Communicated by Prof. Morris, F.Gr.S.) 



This subject has been so recently brought under the notice of the 

 Society that I need not describe the appearances ; but as there seems 

 to be a most extraordinary divergence of opinion as to their cause I 

 have used the general term of " slickensides," meaning " slipping 

 of surfaces." 



In the cretaceous strata of the neighbourhood of Eastbourne these 

 striations may almost be measured by acres, almost every block in 

 the Holywell quarries having its sides either striated or polished ; 

 and in the angles of the blocks, where the chalk has been crushed, 

 the striations assume various directions and curvatures, according, I 

 believe, to the direction of the crushing force. Each surface of the 

 large blocks has one uniform line of striation, though the other sides 

 may be perpendicularly, horizontally, or diagonally striated, as if 

 each block had been moved in various directions. The surfaces of 

 the blocks are more or less embossed ; but the striae enter into all the 

 irregularities, and it often happens that a detached piece of rock lies 

 crushed into a hollow, like a dab of mortar, yet both its external 

 surface and that of the cavity in which it lies are striated by lines 

 in continuity with those of the rest of the surface. Mr. Eordham 

 doubts the existence of horizontal striation ; but since his paper 

 was read I have had the pleasure of showing him these pheno- 

 mena, which have shaken his previous opinion, if not convinced 

 him of the correctness of the slickensides theory. The great 

 difficulty I meet with is to account for the sudden steps or breaks, 

 which give to the surface the fibrous appearance of petrified 

 wood, described by Mantell, but which he attributed to its true 

 cause. In this way I account for the fibrous form, by the 

 disturbance having acted only at short distances ; for it is not on the 

 large surfaces of the blocks that it is found, but in the angles 

 between the blocks, where pieces detached from the adjoining 

 blocks have been crushed into the angles to fill up the vacancy when 

 the blocks settled down again into their former places. 



My attention was first called to this subject by finding an Echi- 

 noderm imbedded in a fissure, with its exposed surface striated ; and 

 soon afterwards I found others crushed, with taluses formed behind 

 their tubercles in the line of pressure. With such evidences of 

 movement in the chalk I began to examine the fissure-, and rock- 

 surfaces ; and I found not only striations and polished surfaces, but 

 calcareous spar, true slickensides, filling up fissures in the chalk, 

 marl, and greensand. I have found exactly the same appearances 

 in the chalk of Brighton and Lewes ; and having examined various 

 rocks (mountain-limestone, coal, serpentine, iron, red sandstone, 

 slate, and surturbrand) and found similar striations, I think it illo- 



Q. J. G. S. No. 121. i 



