CHAPTER II. 



SURFACE GEOLOGY OF THE BENGAL DISTRICTS. 



TL TOST districts of Bengal and Bihar are alluvial. This 

 -^- alluvial plain is a portion of the Indo-Gangetic 

 basin which includes about 300,000 square miles, or Jth of 

 the whole of British India. It is the richest and most 

 populous tract of land, consisting of clay, more or less sandy. 

 Peat, gravels, conglomerate and pure sand occur at intervals. 

 Pistolitic concretions of hydrated iron-peroxide abound in 

 certain regions. In Dinajpur the nodules of iron-peroxide 

 are as big as pigeons' eggs ; but usually they are of the size 

 of peas or even smaller. The alluvium is classified into 

 Old and New. The older alluvium is at a higher level, in 

 the Burdwan Division, in some places, over 100 feet above 

 the sea-level. The newer alluvium occurs near channels of 

 rivers. The Delta of the Ganges and the Brahmaputra is 

 also new alluvium. No marine fossils have been discovered 

 in this alluvium, though in Calcutta a boring down to a depth 

 of 481 feet was made. This boring clearly demonstrated 

 that the surface of the land in the neighbourhood of Calcutta 

 has sunk to a depth of at least 481 feet within the recent 

 geological age. Fresh water shells, pebbles and bits of 

 wood that must have occurred at one time at the surface were 

 brought out by this boring. The greater portion of the 

 Ganges alluvium is Old alluvium containing beds of kankar 

 or carbonate of lime nodules, and of pistolitic concretions of 

 hydrated iron peroxide, On the western edge of the delta of 



