165&,] MORRIS ALLOPHANE. ; If) 



which, meeting below, again separated, and surrounded a large mass 

 of broken chalk ; the allo])hane penetrated the adjacent chalk, which 

 for a short distance on each side was marked by numerous fine ferru- 

 ginous lines arranged parallel to the fissure. 



At the west end of the present workings other fissures are visible 

 (Nov. 1856), which I have occasionally examined during the progress 

 of the works. They extend in the same direction, N.N.E. by S.S.W., 

 dipping about 80° east ; the sides are much striated, and the adjacent 

 chalk is spotted with ferruginous stains. The allophane varies in 

 thickness and texture, being sometimes resinous, or occasionally 

 earthy ; and the flints, which are here sometimes very much crushed, 

 are re- cemented by it. In the vein were patches of a dark powder, 

 consisting of oxide of iron mixed with a little manganese. The 

 allophane following the upper part of the line of fissure is more 

 earthy, about 4 inches in thickness, and is separated from the 

 greenish sands and flints by a thin layer of ochreous clay and impure 

 fuller's earth. 



The allophane is amorphous, investing, and mammillated, trans- 

 lucent or semitransparent, with a somewhat vitreous lustre, but 

 sometimes dull and opake, especially when pure white. 



The colour varies much, either pale lemon or amber, and occa- 

 sionally deep reddish-brown. Its specific gravity is about 2. Frac- 

 ture of translucent masses is brittle ; the dull earthy varieties are 

 conchoidal in fracture, and adhere strongly to the tongue. 



The following analysis is by Mr. Dick : — 



Silica, 18'89 ; alumina, 33'52 ; water, 42*73; lime, 1'67; car- 

 bonic acid, 2*51 ; and a trace of organic matter. 



In ■vde\\ing the circumstances under which this substance occurs, 

 it is evident that it must have been deposited from a fluid or viscid 

 state, not only after the denudation of the chalk and the deposit of 

 the partially abraded flints which are coated with it, and after the 

 accumulation of the ash-coloured or Thanet sand, but subsequently 

 to the disturbance of the whole series, whereby the fissures in the 

 chalk were formed, and in which the allophane is now found. From 

 its chemical composition it is inferred that the formation of the 

 allophane originated in the overlying tertiary strata ; but in what 

 manner is an interesting subject for chemical inquiry, for we cannot 

 always estimate the effects of the long-continued action of chemical and 

 electrical agencies, modified perhaps by temperature, in the ordinary 

 processes of nature. Silicate of alumina is but sparingly soluble ; 

 still the percolation of water, either pure or carbonated, acting upon 

 the previously decomposing clay or clayey sand*, or decomposing 

 them and disarranging the equilibrium of the original substance, may 

 have caused the original or remaining elements to be re-adjusted f, 



* The ash-coloured or Thanet sand consists of argillaceous sand, mixed with 

 numerous grains of a dark green substance, which may have been derived from 

 the previous destruction of some igneous rock containing augite or some other 

 silicate of iron. 



t " Some one element or more," remaiks Dana, " by the influence of external 

 agents, enters into new combinations and is removed ; this disorganizes the 



