576 



Correspondence — Mr. George Maw. 



ON A PECULIAR FOEM OF VAEIEGATION IN CAMBRIAN SLATE. 



Sir, — I send herewith a sketch of a rather unusual form of varie- 

 gation occurring in a slab of Cambrian Slate. I observed it a short 

 time back at Eeading, but am not aware of the exact locality from 

 whence the slate was obtained — probably, however, from the Penrhyn 

 or Llanberis quarries. As it bears on the question of the causes that 

 have induced the parti-colored banding of variegated rocks, you may 

 perhaps give it a place in the Magazine. 



The ordinary form of variegation of Cambrian Slates consists of 

 mechanical nuclei and layers of interbedded matter concentrically 

 environed by pale green slate, the bleaching of which has been due 

 to the abstraction of the greater part of the colouring oxides of iron. 

 In a paper contributed to the Geological 

 Society (Quart. Journal of Geological Society, 

 Nov. 1868), and a communication which ap- 

 peared in the Geological Magazine for 

 March, 1868, 1 endeavoured to shew that this 

 abstraction was independent of the mere me- 

 chanical washing-out in a soluble condition of 

 the colouring oxides. The example repre- 

 sented in the accompanying engraving seems 

 to support this view. Here we have not only 

 a bleached zone immediately adjacent to the 

 central nucleus, but a second concentric zone 

 separated therefrom by an intervening band 

 of unaltered slate. It is obvious that the 

 positions of both the inner and outer pale 

 zones have been determined in some way by 

 the little mechanical nucleus, and if the discolorations were due to the 

 mere solvent action of matter emanating from it, it seems impossible 

 to account for the intervention of the unaltered zone entirely isola- 

 ting the outer pale band from the inner zone and the central frag- 

 ment of foreign matter. Geo. Maw. 



Benthall Hall, Bboseley, 

 November, 1869. 



Quarter Actual Size. 



Successor to the late Mr. Jukes. — Mr. Edward Hull, M.A., 

 F.E.S., F.G.S., has been appointed Director of the Geological Survey 

 of Ireland, and Professor of Geology in the Eoyal College of Science, 

 Dublin, in the room of the late Mr. Jukes. Mr. Hull had not long 

 before been chosen a District-Surveyor on the Geological Survey of 

 Scotland. 



Erratum. — A serious erratum occurred in our November Number 

 (after it had been sent to press). In shifting the form, a line of type 

 — belonging to the Obituary Notice of Dr. Rubidge — was removed 

 by the printer from the foot of p. 527, and inserted at the foot of 

 p. 525, entirely destroying the sense of both the Obituary Notice and 

 of Mr. E. Crais's Letter.— Edit. 



