86 MALLET: GEOLOGY OF DARJILING AND WESTERN DUARS. 



firing. The system of adding the fuel and limestone in alternate 

 layers, and keeping the kiln continuously fed, seems to be unknown. 

 I was told that the average yield per kiln at one operation is about 

 300 mans, and that the sub-contractor who burns the lime receives 

 8 annas a man for it on the spot from the lessee of the quarry. The 

 actual cost of lime-burning in the Darjiling district (including quarry- 

 ing the tufa, cutting wood, &c.,) seems to be about 6 annas a man, 

 but owing to the expense of carriage it was selling this year at 

 Pankabari at Us. 3 a man and at Jallapahar at Us. 2-6. The lime 

 in the latter case came from a place in Sikkim, some miles across the 

 boundary, at an elevation of 3,500 or 4,000 feet. From thence it had 

 to be carried to over 7,000 feet at Jallapahar across the valley of the 

 Ramman, where the elevation is only 1,600 feet. 



The tufa is always nearly pure carbonate of lime, even when the 

 rock from which it is derived is dolomite. Thus analyses of that from 

 the Titi naddi gave the following results : — 





Ordinary 

 porous tufa. 



Crystallin 

 tufa. 



Carbonate of lime ... 



98-10 



98-50 



„ „ magnesia 



1-30 



1-50 



Oxide of iron, alumina, and insoluble matter.., 



•80 



•06 





100-20 



100-06 



Both carbonates of the dolomite are converted into bicarbonates and 

 dissolved by water holding carbonic acid in solution ; but the bicarbonate 

 of lime being the less stable compound is decomposed first, with deposi- 

 tion of tufa, while the bicarbonate of magnesia is carried off in the 

 water. 



The tufa is nearly always porous, the crystalline variety being rare. 

 There is a peculiar species of moss often seen growing on it, which the 

 natives assert turns into, and is in fact the origin of, the tufa. I ob- 

 served what probably gives rise to this idea in the ravine above men- 

 tioned east of the Rehti naddi. The water of the stream is so charged 



( 86 ) 



