16 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 19, 



and new attractions to arise in their then nascent state, by which those 

 elements constituting the allophane may have united and passed oif 

 in solution, and, flowing downward, been deposited among the flints 

 and in the fissures of the chalk below '^. 



Under somewhat similar circumstances to those of its occurrence 

 at Charlton, allophane is met with in the chalk near Beauvais in 

 France, where the lower tertiary sands are superposed on that rock. 



The slight variation in the chemical formula of some of the 

 hydrosilicates of alumina would lead us to infer that several mere 

 varieties in this group have been described as distinct mineral species, 

 the differences of which might be more clearly understood by a 

 careful investigation of the conditions under which these minerals 

 occur in nature, rather than the mere study of them in the cabinet. 



Scarbroitef or kollyrite, another hydrosilicate of alumina, is found 

 in fissures of a grey or ferruginous impure shelly limestone, belonging 

 to the lower oolitic series of Yorkshire, which is covered by the thick 

 series of sandstones and shales containing plants ; it also occurs in 

 the ironstone and sandstone with plants of the same coast. It is 

 allied to pholerite|, a mineral which Mr. Prestwich has detected in 

 the ironstone-nodules of the coal-measures at Coalbrook Dale. 



During the important investigations (under the direction of Dr. 

 Percy, at the School of Mines) in the chemical composition of the 

 difl'erent iron-ores of Britain, a white powder (probably pholerite) 

 was observed, associated with some of the clay-ironstone ; the fol- 

 lowing is its analysis by Mr. Dick : — 



Silica, 41*78; alumina, 36-99; water, 14*26; peroxide of iron, 

 4*51 ; lime, '48; magnesia, "16; alkalies undetermined. 



I have also collected a similar mineral substance occurring on the 

 surface and filling the crevices of the white oolite of Lincolnshire, 

 where this rock is covered by the soft shales, sandstones, and clays 

 belonging to this series. It has probably originated in the same 

 manner as scarbroite and allophane, by the decomposition of some of 

 the mineral substances of the overlying strata. 



original compound, and leaves the remaininp; elements free to combine anew ; 

 such as are capable, consequently unite by their affinities, either alone or with 

 water, or other chemical agents present ; the excess, if any, and soluble, passes 

 off in solution." 



* " Hydrated peroxide of iron decomposes silicate of alumina in such a manner 

 that there is a partition of the silica between the bases, and a double silicate 

 formed. It cannot be supposed that water ever contains hydrated peroxide of 

 iron, but carbonate of iron is almost constantly present. When, therefore, water 

 comes in contact with a mineral containing silicate of alumina, all the conditions 

 for decomposition, whether partial or entire, are present : the protoxide of iron 

 passes by oxidation into peroxide, and this reacts with the silicate of alumina ; 

 while the carbonic acid, liberated at the same time, decomposes other silicates in 

 the mineral." — G. Bischof, Elements of Chemical and Physical Geology, vol. ii. 

 p. 77. Translation by B. II. Paul, F.C.S. (Cavendish Society). 



f See Dr. Murray's notice of this mineral. Twenty-third Report of the Scar- 

 borough Phil, and Archseol. Society, 1855, p. 27. 



X " Of frequent occurrence in crevices of the ironstone-nodules, especially the 

 Penncystoue, in the casts of plants in ironstone," &c. — Geol. Trans. 2 ser. vol. v. 

 p. 487. 



