Correspondence — Mr. David Forbes. 47 



denial : none materially weaken tlie arguments in favour of sub- 

 aerial denudation. 



That I do not take up the matter in detail is owing, not to inability 

 to defend my position, but to a wish to steer clear of controversy. 



W. Whitaker. 



P.S. — (1.) Please insert the following corrections of the second 

 part of my paper which appeared in your November number.' 

 Page 485, fig. 1, the c should have been at the top of the cut. 

 Page 489, fig. 2. The woodcut does not quite agree with the 

 description. The broken lines, above what should have been a firm 

 line on the right and a broken one on the left, but which is con- 

 tinuous and somewhat shaky throughout, ought to have been dotted. 



(2.) In a notice of my '* list of Wells and Borings" (p. 510) the 

 reviewer has mistaken the thickness of the surface-deposits, gravel, 

 etc., given therein, for the depth of the wells. Instead of fifty feet 

 being the greatest depth, some of the wells go down eight times that 

 amount. 



I take this opportunity of asking all who have notes of wells and 

 borings in the London district, to favour me with a copy of them, 

 such information being very useful to the Geological Survey. — W.W. 



RESEARCHES IN BRITISH MINERALOGY. 



Sir, — Your last number (which my absence in Spain has pre- 

 vented me receiving before now) contains a letter from Mr. T. 

 Davies, dated from the British Museum, in which, after referring 

 to some remarks contained in a late paper of mine (Kesearches in 

 British Mineralogy, Phil. Mag. Nov. 1867), he states that true Silver- 

 fahlerz, or Polytelite, is " found in quantity in this country and 

 mined for the silver it contains." 



Being at present occupied in the preparation of a work on British 

 Mineralogy, this information was very acceptable and at once in- 

 duced me to visit the British Museum, in the full expectation of 

 finding so valuable and interesting a British mineral species dis- 

 played in case No. 11 ; unfortunately I could not perceive any such 

 specimen labelled as Silver-fahlerz, or Polytelite, nor any notice of 

 its occurrence in the official guide to the collection. 



In hopes, therefore, of eliciting further information I send you 

 these remarks : — 



Tetrahedrite in general contains more or less silver, but can only 

 be termed Silver-fahlerz, Weissgiltegerz, or Polytelite, when it con- 

 tains a notably large amount of that metal, say a minimum of over 

 5 per cent., for some specimens contain even more than 30 per cent, 

 silver. The external appearance and physical character of this 

 species, do not differ so considerably as to enable the argentiferous 

 or non-argentiferous varieties of Tetrahedrite to be with certainty 

 distinguished from one another. Although the former is generally 

 found to be more brittle, lighter in colour and streak, and to possess 

 a higher specific gravity, chemical examination can alone decide con- 

 ^ Unintentionally omitted from the December Number. — Edit. 



