Correspondence — Rev, 0, Fisher, 147 



pages. Col. Greenwood (whose description of the transportation of 

 flints by the sea is very graphic) has misunderstood me on the sub- 

 ject of residual flints. What I meant was simply that in many 

 chalk districts (not arable-fields) the denudation has been as clean, 

 and as irrespective of flints, as if the groimd had been shaved down 

 with a gigantic scythe. D. Mackintosh. 



POLYTELITE IN CORNWALL. 



Sir, — The substance of Mr. Davies's letter in your last number 

 does not, I imagine, require any reply; but in the postscript he 

 mentions that Professor Church had found 7'23^|o Silver in a crystal- 

 lized fragment of fahlerz, having the density of 4*85, from which I 

 infer that true polytelite is found at that locality. This per centage 

 of silver in, and the specific gravity of, this specimen, might be ac- 

 counted for by supposing the silver in other state of combination, as, 

 for example, argentiferous sulphide of silver (Stromeyerite), which 

 in fracture closely resembles some fahlerz ; and therefore it would be 

 interesting to know from Professor Church whether the other con- 

 stituents of polytelite (antimony, for example) were found, which 

 would at once decide the question. 



Mr. Davies does good service to British mineralogy by directing 

 attention to any cases of unrecorded mineral localities ; and I believe 

 such inquiries will prove that we possess many more mineral species 

 in Great Britain than are at present recorded. Amongst others, I 

 may mention that polytelite from N. Wales, and Gersdorffite from 

 Argyleshire, are described in the second part of my '' Eesearches in 

 British Mineralogy," now in the press. David Forbes. 



THE BOULDER CLAY AT WITHAM STATION, AND THE THAMES 



VALLEY. 



Sir, — My last letter was accidentally printed without my correc- 

 tion, and contains errors, two of which are of some importance. 



In the section, the sand with green-coated flints should be 

 "Thanet" instead of ''Thames" sand. 



My views regarding the age of the " Trail " are singularly mis- 

 represented, where I am made to say it is of " our " age. I wrote 

 " one " age ; which I believe to have been upwards of 110,000 years 

 ago, as I have shown in the fourth volume of your Magazine, p. 197. 



Harlton, Cambridge. 0. FlSHER. 



THE OUSE VALLEY, THE THAMES VALLEY, etc., etc. 

 Sir, — I find that at pages 53-57 of the memoir for sheet 45, 

 reference is made to the Glacial clay, but so slightly that it escaped 

 me. Moreover the Glacial clay tract north of Buckingham, partly 

 traversed by the section in my last letter, is aUuded to (p. 57) as that 

 of the " Oxford or Kimmeridge, as the case may be " ; but as neither of 

 those clays are shown in this part of the map, some slip of the pen 

 may have occurred. Therefore, to this extent, I must qualify the 

 remark in my letter and tender Mr. Green my apology for it. 



