1867.] MAW — CONSOLIDATED BLOCKS. 113 



would be gradually dissolved out by the carbonic acid, and the 

 soluble carbonate of lime would form the cementing medium for 

 the residue. 



When it is borne in mind that the soluble and insoluble conditions 

 of carbonate of lime are easily interchangeable through different 

 proportions of the acid, the cementing together of Drift containing 

 calcareous matter, by springs charged with carbonic acid, is readily 

 explained ; the only difficulty with respect to the blocks in the Cod- 

 denham drift is the isolated and unequal way in which the process 

 has acted. In such open porous gravels, water charged with carbonic 

 acid would freely percolate through the entire mass ; and it seems 

 difficult to explain the consolidation taking place in weU-defined un- 

 connected patches, unless it results from the local discharge of car- 

 bonic-acid gas at particular points. 



The whole of the concrete masses at Chopping' s Hill Farm were 

 readily resolvable into sand and gravel by the action of hydrochloric 

 acid, proviag that they were held together by a calcareous cement. 



The block before referred to, resting on the B,ed Crag near "Wood- 

 bridge, contained no carbonate of lime, but a small quantity of lime 

 in the form of silicate. The following is an analysis by Dr. 

 Yoelcker ; — 



Analysis of Saccharoid Block resting on the Bed Crag near 

 Woodhridge. 



Lime in a state of silicate 0*41 



Oxides of iron and alumina 3'04 



Silica and loss 96-55 



100-00 



A Sarsen Stone from Avebury, Wilts, contained a little free car- 

 bonate of lime, but otherwise showed a composition somewhat simi- 

 lar to that of the block from Woodhridge : — 



Analysis of Sarsen Stone from Avehury, Wiltshire. 



Oxides of iron and alumina 0-29 | soluble in dilute 



Carbonate of Lime 0-46 J hydrochloric acid. 



Alumina 1 . i. i. jy ^^' i. f 0-58 



Lime jinastateofsihcate...|().gg 



Silica 98-11 



100-00 



The composition of both of these stones indicates the presence of 

 from two to three per cent, of sihcate of lime and alumina, which 

 appears to hold the siliceous particles together. 



It is probable, as in the case of the concrete masses at Chopping'^ 

 TTill Farm, that carbonate of lime was the original cementing 

 material, and that hme in solution in contact with the silica, and a 

 smaU quantity of alumina, would form a thin coat of silicate of 

 lime and alumina round each particle of sand, and agglomerate 



