MINBU. 



45 



With regard to the general shape of the mud volcanoes we may 

 distinguish two types, vis. : — 



(a) Basins. 



(b) Cones. 



In their extremes the two types are well distinguished, and no 

 greater contrast could be imagined than that which exists between 

 mad volcano No. 5 (see plates III and IV) and one of the wells 

 in the northern group, but No. 1 of the southern group proves dis- 

 tinctly that these are simply different effects of one and the same 

 cause, an explanation of which will be given presently. 



(a) Basins. — The basins are more or less circular or elliptical 

 holes formed in the ground. When dry it may be seen that their 

 walls are steep and the bottom rounded. When in activity 

 the basins are filled with muddy water, which generally forms 

 a sediment ; from the bottom rise incessantly numerous small 

 gas bubbles which give the liquid the appearence of boiling. Some 

 of the vents seem rather constant, as indicated by the rise of the gas 

 bubbles ; others are only temporary ; the gas rises for some time at 

 a certain spot, the vent becomes clogged and fresh gas bubbles 

 rise at another place. Petroleum is brought up with the gas 

 and forms a more or less continuous film on the surface of the 

 water. 



Although most of the basins are permanently in a state of activi- 

 ty, they never produce mud streams of any extent. This applies 

 particularly to the northern group. The general shape of the great 

 well No. 1 has not altered since 1888, nor has that of any of the other 

 wells, for instance, No. 7 of the southern group. 



(<5) Cones. — The cones are built up of mud streams, generally with 

 very steep sides, as may be gathered from any of the plates, 

 illustrating them. We may assume, that before the steep upper cone 

 was finished, the vent was situated on a low flat cone such as is 

 represented mud volcano No. 1. The conditions for the raising of a 

 cone seem to be the presence of a great mass of viscous mud 



( 9i ) 



