94 Correspondence — Dr. C. Callaicay — Prof. T. G. Bonney — 



on both sides of the question ; my object, however, in writing this, 

 was not to discuss the origin of the Chalky Boukiei'-clay, but to 

 point out that the more remarkable disturbances in the Chalk near 

 Norwich are of glacial origin, and subsequent to the deposition of 

 the Norwich Crag. The section at Litcham, described by Mr. S. V. 

 Wood, jun., tells the same story ; and having visited the Bluffs at 

 Trimmingham on many occasions with my colleague, Mr. Clement 

 Eeid, I have been led to adopt his explanation that the disturbances 

 of the Chalk there were produced by land-ice. 



Fakenham. Horace B. Woodward. 



THE PEE-CAMBRIAN ROCKS OF BRITAIN AND BOHEMIA. 

 Sir, — In Mr. Marr's valuable paper On the Pre-Devonian EocJcs of 

 Bohemia, published in the last number of the Geological Society's 

 Journal, there is one point on which further evidence would seem 

 desirable. I refer to his correlation of the Bohemian gneissic series 

 with the St. David's Dimetian. He describes the Bohemian rocks 

 as " gneiss," " gneissic rock .... interspersed with small garnets," 



"white foliated quartzose rock," "crystalline limestone 



strongly foliated, and containing silvery mica." Besides these rocks 

 there is a "band of graphite" and dykes of "black eclogite." 

 Having examined the Dimetian of St. David's from top to bottom, 

 I did not find any one of the varieties named by Mr. Marr. The 

 series is mainly composed of quartzite and granitoid rock, and the 

 existence of foliation has not been proved in either the quartzose or 

 the more felspathic types. I do not deny the Dimetian age of the 

 Bohemian gneiss, but 1 should hesitate to accept the present evidence 

 as decisive of the point. From Mr. Marr's description, the Pebidian 

 age of etage A appears highly probable, and the discovery is of great 

 interest. The two Pre-Cambrian groups in Bohemia are in their 

 lithology not unlike the two Anglesey series, of which full descrip- 

 tions will shortly be communicated to geologists. If the older 

 Anglesey series could be definitely accepted as Dimetian, Mr. Marr's 

 opinion would receive strong confirmation. C. Callaway. 



"Wellington, Salop, Nov. 30, 1880. 



ON THE TUSCAN SERPENTINES. 

 Sir, — The author of the notice of Prof. Pantanelli's paper IBiaspri 

 della Toscana, etc. (Geol. Mag., 1880, p. 564) inadvertently attributes 

 to me an opinion which I do not hold, when he includes me among 

 those who have recently maintained " that the (Tuscan) serpentines 

 represented true submarine lavas of the Upper Eocene." On the 

 contrary, in my paper (Vol. YI. p. 362) I am at some pains to show 

 that these serpentines are intrusive in the diaspro, etc. The evidence 

 against their being contemporaneous lava flows is strong. It is a 

 remarkable thing that olivine rocks appear very rarely to reach the 

 surface. I have never myself seen a serpentine which was not 

 intrusive. Some pierites, however (e.g. that described by Professor 

 Geikie in his paper on the Volcanic Eocks of the Firth of Forth), 

 and limburgites appear to be lava flows, as may possibly be one or 

 two other olivine rocks. T. G. Bonney. 



