DYKES SALSES. 



-where the lava is less fluid than in that of Stromboli, new cones 

 are sometimes formed in the midst of the crater, which first rise 

 in the form of a dome, and then burst out so as to form a small 

 -active volcano in the middle of the crater of the great one. This 

 phenomenon is often presented within the crater of Vesuvius 

 '(fig. 67) and was more particularly witnessed in 1829. 



155. Sometimes the lava, which is pressed upwards instead of 

 being violently ejected, spreads itself in a sheet of greater or less 

 thickness over the surface (fig. 68) where it hardens, and is subse- 

 quently covered by other deposits. Cases have been found also 

 where a succession of these strata, formed at different intervals, 

 with interposed strata of other matter, have been observed (fig. 69). 

 In such cases the matter forming the superior stratum is seen to 



Fig. 68. 



Fig. 







Fi *- 70 - 



;have passed in a liquid state through the inferior strata pre- 

 viously described. 



156. It .of ten happens that the lava is solidified in clefts, which are 

 nearly vertical, and thus forms 



walls, called dykes, which fre- 

 quently rise to the surface. 

 In such cases the solidified 

 lava, being much harder and 

 less susceptible of degradation 

 from atmospheric and aqueous 

 influences, remains standing 

 when the matter surrounding 

 it is swept away, and thus 

 forms a wall rising above the general surface (fig. 70). 



157. The phenomena of Salses, or mud volcanoes, has been 

 already briefly noticed in our Tract upon " Earthquakes and 

 Volcanoes," Vol. IV. p. 169. The mud volcanoes are characterised 

 also by the conical form, but their cones are much less elevated, 

 their slopes being flatter (fig. 72). They have at their summit a 

 crater-formed cavity frequently filled with liquid mud, on which 

 large bubbles are continually formed which, bursting, scatter 

 around them earthy matter. There are sometimes, over a surface 

 of little extent, a great number of these cones in full activity, 

 some of which have a height of ten or twelve yards. Sometimes 

 such an assemblage of cones is found at the summit of a mound 

 from fifty to two hundred yards in height, formed of argillaceous 



i 2 115 



