ORIGIN OF LAKE BASINS. 23 



Bombay. This remarkable crateriform lake, known as Lonas lake, is 

 described by K. D. Oldham ^ as follows : 



" The surrounding country for hundreds of miles consists entirely of 

 Deccan trap, and in this rock there is a nearly circular hollow, about 300 

 to 400 feet dee[) and rather more than a mile in diameter, containing at the 

 bottom a shallow lake of salt water without any outlet, whose waters 

 deposit crystals of sesquicarbonate of soda. The sides of the hollow to 

 the north and northeast are absolutely level with the surrounding 

 country, while in all other directions there is a raised rim, never exceed- 

 ing 100 feet in height and frequently only 40 or 50, conqjosed of Ijlocks 

 of basalt, irregularl}- piled, and precisel}^ similar to the rock exposed on 

 the sides of the hollow. The dip of the surrounding traps is always 

 from the hollow, but very low. 



" It is difficult to ascribe this hollow to any other cause than volcanic 

 explosion, as no such excavation could be produced by any known form 

 of aciueous denudation, and the raised rim of loose blocks around the edge 

 appeai-s to preclude the idea of a sinq^le depression. It is true that there 

 is no sign of any eruption having accompanied the formation of the crater ; 

 no dyke can be traced in the surrounding rocks ; no lava or scoriae of later 

 age than the Deccan trap period can be found in the neighborhood. The 

 raised rim is very small, and cannot contain a thousandth part of the rock 

 ejected from the crater, l)ut it is impossil)le to say how much was reduced 

 to fine powder and scattered to a distance, or removed by denudation. 



" Assuming that this extraordinary hollow is due to volcanic explosions, 

 the date of its origin still remains to be determined. That this is long 

 posterior to the epoch of the Deccan traps is manifest, for the hollow, 

 appears to have been made in the present surface of the country, carved 

 out by ages of denudation from the old lava flow. To all appearance 

 the Lomis lake crater is of comp;iratively recent origin, and if so it 

 suggests that, in one isolated spot in India, a singularly violent ex})losive 

 action must have taken jdace, unaccompanied by the eruption of melted 

 rock. Nothing similar is known to occur elsewhere in the Indian 

 Peninsula."' 



liesides the obstructions to drainage produced l)y overflows of lava, 

 and by volcanic explosions, it may also be noted that volcanic dust and 

 ashes, ejected from volcanoes during times of violent eru[)tions, may be 

 deposited over the adjacent country in such a manner as to choke the 

 streams and possibly form dams which woidd ri'tain lala-s. This process 

 1 " A Manual of tlie Geoloiry of Iiulia." iM od. Calcutta, IS!)."., pp. 1!>, 20. 



