Correspondence — Mr, W, Stephen Mitchell. 295 



crust of the earth, and seems to bear out the views of the late D. 

 Sharp, on the direction of slaty cleavage. 



The applicability of this illustration was first suggested to me in 

 a note from the Rev. 0. Fisher, referring to my recent letter in the 

 Magazine, on " Gravitation and Plorizontal Compression," in which 

 he observes, " I find that if you take into consideration a spherical 

 shell, of moderate, say a few miles, thickness, and conceive it for a 

 moment unsupported by the matter within, then the horizontal pres- 

 sure upon any two sides of a cubical element of this shell will be 

 equal to the weight of a column of rock of the same density and 

 half the length of the earth's radius. This would be sufficient to 

 crush any strata, and is, I believe, the force to which the elevation of 

 mountains is due." 



If you also take into consideration the effects of even the slightest 

 inequality of local horizontal expansion, due to heat, its resolution 

 vertically, in an arched form (bulging), would account for the 

 fullest amount of displacement observed in the earth's crust. Take 

 a segment of, say, only a hundred miles ; an expansion of but j^ 

 part of its length would produce a vertical elevation of several 

 hundred feet at its centre. 



The late D. Sharp's observations (Quart. Journ. of the Geol. Soc, 

 vol. iii., p. 74,) tend to show the relation between the dip of slaty 

 cleavage to areas of elevation in its apparent radiation from the 

 axis of upheaval. If the slightest abnormal expansion is super- 

 added to the uniform horizontal pressure within a sphere due to 

 gravitation, it appears probable that the direction of the force would 

 determine the dip and direction of cleavage plains. 



As Mr. Fisher informs me he has recently communicated a paper 

 on a kindred subject to the Cambridge Philosophical Society, I for- 

 bear, till it appears in print, to do more than give the drawing of 

 the displaced wall-coping in further illustration of the suggestion I 

 threw out in the March number of the Magazine. George Maw. 



JBenthall Hall, Broseley. 

 May 2nd, 1868. 



FOSSIL PALM-LEAF FEOM THE EOCENE OF THE ISLE OF WIGHT. 



Sm, — In Room I., Wall-case 6, of the Geological Gallery of the 

 British Museum is a fossil Palm -leaf in a nodule to which the follow- 

 ing label is attached : — " Flahellaria lamanonis, Brogn. Eocene, 

 Isle of Wight. From Dr. Mantell's Coll"- fig'i- at p. 52 of Mantell's 

 fossils of the Brit. Mus. 1851." The locality given in Dr. Mantell's 

 book is White Cliff Bay. On the back of the specimen is written in 

 pencil " Upper Bembridge or Lower Hempstead." 



Can any of your readers state the exact locality and bed from 

 which this specimen came, and whether any other specimens have 

 been found in White Cliff Bay ? 



May 14, 1868. W. Stephen Mitchell. 



