Correspondence— Rev. 0. Fisher. 383 



Sandhurst ; gentlemen who have proved that they are qualified to 

 give accurate descriptions of the topography and geology of the 

 country. 



E. Bkough Smyth. 

 Melbourne, l%th May, 1871. 



CONCKETIONAET STEUCTURE IN PLASTER. 



SiK, — The concretionary structure in plaster noticed by your 

 correspondent Benwyan has been frequently noticed by me, as it 

 must have been by many. I do not think that the explanation given 

 by him, that it results from segregation or crystallization, can be the 

 true one ; for if that were so, the nuclei of the concretions should be 

 inside the plaster ; whereas I think they will be observed to be 

 external. The concretions resemble saucers nested, rather than 

 spherical shells nested. I have been used to attribute the appearance 

 to the habit that plasterers have of casting the mortar on with a 

 dash, so that it spreads from a central spot in concentric waves 

 around. Thus the particles of the mortar are arranged in shallow 

 saucer-shaped layers. And possibly, owing to some mechanical law 

 in the distribution of the pressure from the central spot towards the 

 periphery of the lump thrown on, the density of the mortar may 

 alternately be greater and less in successive layers ; and it is even 

 possible that an arrangement of the particles analogous to cleavage 

 may be produced. The subsequent passage of the smoothing tool 

 over the whole obliterates the structure superficially, but time 

 reveals it again by the process of weathering. 



0. FiSHEK. 



DENTJDATION OF THE SHROPSHIRE COAL-FIELD. 



We are favoured by Mr. John Eandall, F.G.S., of Madeley, Salop, 

 with a lengthened criticism upon a paper by Mr. Daniel Jones, 

 F.G.S., " On the Denudation of the Coalbrook-dale Coal-field," pub- 

 lished at p. 200 of our May number.^ Mr. Eandall states that the 

 conclusions arrived at by Mr. Jones are identical with those of Mr. 

 Scott, Sir E. Murchison, and himself as unmistakably shown by 

 the sections published by Mr. Scott (Quart. Journ. Geol. Soc, 1861, 

 vol. 17, p. 457), and that Mr. Purton has since figured the same 

 thing (see Geological Magazine, 1865, Vol. II., p. 515). 



Mr. Eandall always held the opinion " that denudation took place 

 prior to the general elevation of the Coal-field and the great faults 

 by which it is intersected." The quotation from Mr. Eandall's 

 letter to the Mining Journal, given by Mr. Jones, refers to "one 

 particular case on the eastern boundary of the Shropshire Coal-field, 

 showing evidence of denudation and disturbance combined." 



" The case rofers exclusively to the Coals in the Halesfield and 

 Kemberton pits, and not to the general question." 



1 Its earlier appearance has been delayed from -want of space, and, indeed, we 

 cannot now give Mr. Randall's criticism in full. 



