23 LA TOUCHE: GEOLOGY OF WESTERN RAJPUTANA. 



to west, and from Pokaran in the north to Jalor in the south, a distance 

 of 120 miles. The area over which they are found thus amounts to 

 about 17,000 square miles. They have hitherto not been identified with 

 certainty beyond the limits of the Jodhpur State, though there is no 

 doubt that rocks closely resembling them occur in other parts of India. 

 Some similar rhyolites, occurring at Tusham hill, about 240 miles to 

 the north-east of the nearest outcrop of the true Malanis near Jodhpur, 

 have been described by Colonel McMahon. 1 



The conditions under which the lava sheets are now exposed, in iso- 

 lated hills and ridges often separated by miles of sand-covered ground, 

 render it impossible to make out any regular sequence among them, 

 nor can any estimate be formed of the total thickness of the volcanic 

 series. There are no indications whatever of the presence of the vents 

 from which the lavas were poured out, except in one doubtful instance 

 which will be referred to subsequently. 2 The slight amount of distur- 

 Dance they have undergone has not been sufficient to bring the base of 

 the series to the surface except in one locality, 3 and the vents, 

 whether these were in the form of fissures from which the lavas welled 

 out and consolidated on either side, or pipes of the Vesuvian or Puy 

 type, are buried beneath the accumulation of lava, or may perhaps be 

 concealed beneath the sand. If the vents were filled towards the 

 close of the volcanic period with loose material, it is quite likely that 

 they would have suffered denudation more rapidly than the hard sheets 

 of lava surrounding them, and their sites would be covered with alluvium 

 and sand. With regard to the nature of the vents, there does not 

 appear to be any concentric arrangement of the lava sheets, such as, 

 even in the fragmentary state in which they are exposed to view, we 

 might expect to find if they had been ejected from a centre of eruption, 

 and it is therefore more probable that they were of the fissure type. 



1 Rec. Geol. Suiv. Ind., Vol. XVII, Pt. 3, p. iol. Vol. XIX, Pt. 3, p. 101. 

 = Infra, p. 51. 

 * Supra, p 19. 



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