380 Notices of Memoirs — Indian Geology. 



generally be met with. The older of these may be either marine 

 (estuary) 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. Whilst no very great thick- 

 ness of the newer deposit 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. 



Every river that discharges its waters into the sea, does so under 

 a condition either of subsidence, of quiescence, or of elevation, and 

 to what extent this condition influences the character of the deposits 

 as well as the physical peculiarities of the delta, Mr. Theobald en- 

 deavours to illustrate by the Irawadi, and the contrast which its 

 delta presents to that of the Ganges. 



These two — the Ganges and Irawadi — present examples of rivers 

 subjected to, respectively, the first and last-named conditions. At 

 Fort William boring operations for an artesian well showed 70 feet 

 of the newer (Gangetic) alluvium, resting on the denuded surface of 

 the kunker clay, which is indicated by the rolled kunker pebbles 

 found at the junction. The author regards the kunker clay as an 

 estuarine deposit (the older alluvium) accumulated during an upward 

 movement of the land. It is a stiff clay, of a homogeneous character, 

 and of a mottled yellow or pale buff colour, reddening by exposure. 

 The kunker is disseminated through the clay, sometimes in well- 

 defined nodules, more often in irregular stringy courses. 



The newer alluvium comprises a very varied series of beds directly 

 deposited by the waters of the Ganges or its tributaries. They con- 

 sist of sand sometimes dark brown, at others a dazzling white, with 

 several beds of peat. It is at once an extensive and important group 

 deposited within the trough excavated by the Ganges in the older 

 clay, or filling up such low-lying tracts as receive the flood-waters 

 of the Ganges during its annual inundations. Mr. Theobald is of 

 opinion that the area is undergoing depression at a rate which is 

 adequately counterbalanced by the accession of Gangetic sediment 

 on the surface^ 



Mr. Theobald then compares the alluvial deposits of the Ganges 

 valley with those of the Irawadi, prefacing the subject with a few 

 remarks on the physical character of the country, which presents 

 some peculiar features. 



The alluvium of the Irawadi belongs almost entirely to the older 

 group, and the cause of this is attributed to the fact that the delta 

 of this river is at the present time in precisely the same condition as 

 was the delta of the Ganges when the first layers of its alluvium, 

 70 feet below the present surface at Calcutta, were being deposited. 

 At this period the older marine group had become sufficiently raised 

 to admit the deposition of beds stamped with a fluviatile and terres- 

 trial character, and even the accumulation of such matters as peat ; 

 since then a steady down water movement of the Gangetic delta has 

 permitted the enormous accumulations of newer alluvium, which 

 covers so large an area in Bengal. No such movement has hitherto 



