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Cii XXVIL] NxVTUEE OF SUBTERRANEAN IGNEOUS ROCKS. 77 



hills occur to the westward of the Hara Momitams and the 

 river Hubb. (See Map, p. 98.) One of the cones is 400 

 feet hio-h, composed of light-colonred earth, and having at 

 its summit a crater 30 yards in diameter. The liquid mud 

 which fills the crater is continually disturbed by air-bubbles, 

 and here and there is cast up in small jets."^ 



Fig. 100. 



Mud cones and craters of Hinglaj near Beila, district of Lus, 120 miles north-^^•est 

 of mouth of Indus. From original drawing by Capt. Robertson. (See Map. p. 98.) 



Frequency of eruptions, and nature of subterranean igneous 

 roc/i;s.— When we speak of the igneous rocks of our own 

 times, we mean that small portion which, in violent erup- 

 tions, is forced up by elastic fluids to the surface of the 

 earth,— the sand, scorise, and lava, Avhich cool in the open 

 air. But we cannot obtain access to that which is congealed 

 far beneath the surface under great pressure, equal to that 

 of many hundred, or many thousand atmospheres. 



* SeeEuist,Volcanos of India, Trans. Captain Robertson, Journ. of Roy. 

 Bombay Geol. Soe. vol. x. p. 154, and Asiat. Soc. 1850. 



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