138 MANGANIFEROUS NODULES IN BOULDER-CLAY. 



The manganese peroxide was estimated in the usual manner. 

 The weighed substance was placed in a small flask with some 

 hydrochloric acid and heated. The chlorine given off was led 

 through some bulb tubes containing potassium iodide solution 

 and the amount of iodine liberated was estimated by deci-normal 

 thiosulphate solution. 



Later, a second sample was taken and subjected to a similar 

 analysis. The following are the percentage results of the exam- 

 ination of the two samples : — 



Moisture . . . . 



Silica . . . . 



Oxide of iron 

 Manganese peroxide 

 Manganese protoxide 

 Calcium oxide 

 Phosphates, carbonate, &c. 



From the above figures it will be seen that the nodules 

 differ in corrposition, some being richer in manganese, and the 

 manganese being in some in a more highly oxidised condition 

 than in others. The nodules appear to be formed of a silicious 

 material cemented together by the oxides of iron and manganese 

 with some phosphate and carbonate of calcium and a quantity 

 of moisture varying in proportion to the iron present, since 5'6 

 bears the same ratio to 23-9 as 7 does to 2g*g. 



It is remarkable that the manganese protoxide and lime in 

 the first sample are exactly equivalent to the protoxide and lime 

 in the second sample, in fact, the two oxides have evidently been 

 able to replace each other in the nodules. 



This somewhat remarkable material has a great similarity to 

 that already described to the Essex Field Club in a paper read 

 by Mr. T. S. Dymond at the meeting held Dec. nth, 1897 

 (Essex Naturalist, Vol. X., pages 210-12).' This paper 

 referred to the hard masses of cemented gravel found in some 

 parts of Essex, chiefly round Tendring, and which were used for 

 building purposes. The cement proved to be "ferric oxide 

 mixed with the peroxide and protoxide of manganese, together 



I Reference should also be made to the paper by Mr. Johnson on a Manganiferous Seam 

 in the Drift at Ilford, print^id in the present part of EssKX Naturalist, which was received 

 by the Editor previous to the reading of Miss Tliresh's paper. — En. 



