190 ECONOMIC GEOLOGY. [Pt. III. 



Bearing in mind the small quantity of ash in the brighter portions 

 °f the seams, it will be at once evident that the quantity in the duller 

 portions must be considerable. It is at least 20 to 30 per cent, so that 

 these duller portions are evidently nothing more than an extremely 

 carbonaceous shale. Hence the value of any coal from Raniganj, or, 

 so far as is known from any Damiida rocks, depends mainly upon the 

 proportion of bright lamince contained in it.* 



The presence of this carbonaceous shale is the main impediment to coke 

 being manufactured fromDamuda coal. It is too impure to soften by heat. 



The drawbacks to the universal employment of Raniganj coal, and 

 the reason why, despite its greater cost, English coal is still generally 

 employed for many purposes in Bengal, and especially for sea-going 

 steamers on long voyages, may be briefly summed up in the following : — 



1st. The non-coking property of Raniganj coal. 



2nd. The small proportion of fixed carbon. The value of a coal 

 for heating purposes varies very nearly as the amount of fixed carbon 

 contained in it. 



3 re?. The large proportion of ash. This and the last mentioned disad- 

 vantage may be briefly summed up by stating that Raniganj coal gives a 

 much lower " duty" than any good quality of English coal, and, con- 

 sequently, a larger quantity is required to do the same amount of work. 



4th. Its liability to spontaneous ignition. The first three objections 

 to the use of Indian coal need no further remarks. The liability to 

 spontaneous combustion is mainly due to the large quantity of iron 

 pyrites in the coal, and it appears probable that, as the proportion of 

 pyrites varies very much in different seams, coal may be found, by 

 careful selection, to which this objection will not apply, especially if 

 care be taken that the coal is shipped fresh from the mine, and that 

 it is not exposed to the action of moisture. 



* The above is, more or less, the case with all coals, but the laminae are far more marked 

 in the coals of India than in those of Europe. 



