Corre-<ipondence — Mr. A. G. Cameron. 91 



Wern-y-scadog, Llanfyllin, was described as P. ttimida, and a Tham- 

 niscus from the volcanic ash of Middleton Hill, near Welshpool, 

 probably of Bala age, as T. antiquns, both from a collection sent by 

 Mr. J. B. Morgan, of Welshpool, to Prof. Lapworth for identi- 

 fication. A list of the species of PhjUopora, hitherto described from 

 Lower-Silurian beds, and of both Upper and Lower Silurian forms 

 of Thamniscus, was added, and the relations of the various known 

 species to those described in the present paper were discussed at 

 some length. 



coisiaEsiFOZsriDiBnriTOiB. 



FULLERS EAETH AND WATER SUPPLY. 



Sir,— At Woburn, Beds, the Fullers earth is obtained by digging 

 cylindrical holes or wells, as they are there called, in the Greensand, 

 until this marl is reached. Sometimes there is water, oftener not ; 

 but when there is, it is the finest and sweetest in the country, very 

 clear, never failing, but not very abundant. So good is it that those 

 domestic wells deriving their supply from some other source than 

 the Fullers earth, are treated to it, from time to time artificially, by 

 having masses of it placed in them. The cleansing properties of 

 this 'earth,' as applied to blankets, etc., is universally recognized, 

 but I never heard before of its being used for cleansing water 

 supply. I understand, however, that such is the case, the Woburn 

 earth having been sent into a neighbouring county for that purpose ; 

 though I have not as yet been able to obtain corroboration of this, 

 nor to find out whether it is put into filter beds with the other 

 material, or in what way it is made use of. A. G. Cameron. 



H.M. Geological Survey, Bedford. 



THE PHENOMENA OF STRAINS, Etc., OBSERVABLE IN OBSIDIAN. 



Sir, — In the August number of the Quarterly Journal of the 

 Geological Society, Mr. F. Rutley describes the phenomena of strain 

 in the glass of some obsidians around embedded crystals. I enclose 

 a photograph of similar depolarizing effects in a slide, cut from a 

 specimen of Mexican obsidian given to me some years ago by my 

 friend Mr. J. Backhouse of York, among a number of pieces several 

 of which showed the phenomenon in qxiestion. 



It is rather remarkable that in the same slide some of the crystals 

 exhibit the luminous brushes as described and figured in the article 

 cited, while others apparently have no efi"ect upon the state of the 

 glass. This is the case round both the crystals of felspar of various 

 kinds, and the black specks, mostly rounded, which I suppose are 

 magnetite. 



I determined that the glass is compressed by observing the eff"ects 

 produced by the interposition of different parts of a strip of glass 

 between the slide and the objective when bent by the pressure of the 

 fingers in a horizontal plane. By this means it was possible to so 



