464 The Geological Survey of India. [Oct., 



Comparisons have recently been drawn between the Alluvial 

 deposits of the Irrawadi and the Granges. Every river that dis- 

 charges its waters into the sea has the character of its deposits 

 influenced according to whether the area be in a state of subsidence, 

 quiescence, or of elevation. Generally in every large river-basin 

 two distinct alluvial deposits will be met with. The older of these 

 may be either marine (estuarine) or fluviatile (lacustrine), or of a 

 mixed and alternating character ; but the newer group is essentially 

 fluvio-lacustrine, and directly produced by the existing river. While 

 no very great thickness of the newer stratum can anywhere have 

 been deposited without a corresponding subsidence of the area, a 

 very large accumulation of the older or estuarine deposit may have 

 taken place during an elevation of the area covered by it. 



The Granges and Irrawadi present examples of rivers subjected, 

 respectively, to the former and latter conditions. The alluvium of 

 the Granges, as ascertained from a well-boring at Fort William, con- 

 sists of 70 feet of the newer or fluviatile deposit, resting on the 

 denuded surface of the " kunker clay." This clay is regarded as an 

 estuarine deposit accumulated during an upward movement of the 

 land. The Grangetic area is now considered to be undergoing de- 

 pression at a rate adequately counterbalanced by the accession of 

 sediment brought down by the river. The alluvium of the Irrawadi 

 belongs almost entirely to the older group, this river-delta being at 

 the present time in precisely the same condition as was the delta of 

 the Granges when the first layers of its alluvium, 70 feet below the 

 present surface at Calcutta, were being deposited. The difference 

 in the fertility of the two areas is attributed to the greater richness 

 of the newer alluvium, and hence the inability of the delta of 

 the Irrawadi to compare with that of the Granges in agricultural 

 produce. 



The geology of the neighbourhood of Madras is noticed in the 

 third volume of the c Eecords.' The greater part of this district is 

 occupied by rocks of Secondary, Tertiary, and Eecent ages, the re- 

 mainder is taken up by metamorphic rocks, forming part of the great 

 gneissic series of Southern India. Some time previously, beds of 

 magnetic iron-ore were pointed out in the metamorphic gneiss rocks 

 of the Madras Presidency, the supply of which was considered to be 

 practically inexhaustible. 



The Eajmahal plant-beds consist of conglomerates, sandstones, 

 gritty clays, and shales. 



The Laterite deposits are also pointed out. They comprise 

 clayey conglomerates, gravels, and sands, which graduate one into 

 the other. The gravels contain pebbles of quartzite and gneiss, 

 mixed with pisiform ferruginous pellets. Other deposits called the 

 Conjeveram gravels are noticed ; they differ from the laterite beds 

 in the absence of ferruginous matter. Both appear to contain imple- 



