480 Mr. C. Tomlinson on the Weathering of Rocks, 



that there is no room for the circulation of water, &c, a 

 formation like those near the Cheese Wring will be effected ; 

 or if a number of slabs be superposed and the lower ones be 

 more exposed to the weather, so as to catch the dripping and 

 drifting water from the upper, we get a formation like the 

 Cheese Wring, each slab being worn away at the edges, and 

 the lower ones much more than the upper, so that if it does 

 not topple over it may in time end in a Rocking Stone *. 



Another of Mr. Haggard's statements, given as if from 

 actual observation, seems to be nearly as erroneous as in the 

 former cases. He describes a bare strip of veldt as being " a 

 very dangerous spot in a thunderstorm, but a great safe- 

 guard to the house and trees around it ; for the iron-stone 

 cropped up here, and from the house one might generally see 

 flash after flash striking down on to it and even running and 

 zigzagging about its surface." (Page 104.) 



In connexion with this statement two questions arise. 

 First, is it really iron-stone, or an ore of iron that crops up at 

 the surface, and if so, secondly, would the line of least resist- 

 ance be from the cloud to this particular spot, so that the 

 lightning might generally be seen zigzagging over its 

 surface ? 



As to the first question, Mr. Penning, in describing the 

 high-level coalfields which cover the eastern portion of the 

 Transvaal, saysf: — " The shales and the trap-rocks of course 

 weather very differently, the shales with a flat curve and 

 rounded outline, the igneous rocks, when imbedded, forming 

 steep krantzes, or precipices ; when occurring as dykes, with 

 a rough stony outcrop, resembling long lines of waterworn 

 boulders. The rounded form of the loose stones is due to 

 concentric decomposition and weathering ; this feature and 

 their brown, frequently glazed, coating, have together given 

 rise to two wide-spread errors — that they are iron-stones, and 

 that they are waterworn." 



In the second place, let us consider for a moment the con- 

 ditions under which a disruptive discharge takes place during 



* British Association Keport, Norwich, 1868. Trs. of Sections, p. 65. 

 Camphor is sometimes sold in neatly cut parallelopipeds. One of these 

 exposed to the air of my room during several months, first lost its solid 

 angles, then its edges by evaporation, and so passed into a well-shaped 

 oblate spheroid. Two or more of these parallelopipeds piled on each 

 other and exposed to the air, indoors and out, and also to various tem- 

 peratures and to solvents, gave some interesting results illustrative of 

 weathering ; but the details are too lengthy for a note in this place. 



t Quart. Journ. Geol. Soc. xl. p. 662 (1884). 



