STRATICRAPHICAL FEATURES. 2 J 



as bright red, blue, green, dark grey or black are common, and a few 

 are white or light grey. Sometimes the porphyritic crystals are 

 altogether absent. The lavas are all highly siliceous and so hard that 

 the unweathered surface cannot be scratched by a knife. In spite of 

 the great age of these rocks, and of any alteration they may have 

 undergone, there is clear evidence that when originally formed they 

 were true volcanic ejectamenta, spread out over the surface of what 

 was probably dry land or the shores of a shallow lake or sea. All the 

 well known characteristics of glassy lavas can be observed in them. Some 

 of the flows show beautifully developed flow- structure, and in many 

 the original glassy texture has hardly undergone any alteration, so that 

 a thin slice remains almost dark under the microscope between crossed 

 Nicols. Perlitic structure, one of the most certain indications that 

 the rock was originally a true glass, can be detected in a few cases, and 

 sphserulitic structure is very common, ranging from the most micros- 

 copic examples up to nodules an inch or more in diameter. A vesi- 

 cular structure is not often met with, but it does occur in some of the 

 flows ; the vesicles are often filled with secondary minerals, forming 

 amygdales. 



In many places the lavas are interstratified with thick beds of tuff 

 and breccia formed out of the dust and fragments of the lavas them- 

 selves, and evidently due to volcanic explosions. Some of the tuff beds 

 show signs of having been rearranged under water. But the most con- 

 clusive evidence for the subaerial character of the volcanoes is afforded 

 by the presence of bands of conglomerate, formed of well rolled 

 pebbles of the lavas, intercalated between the flows in many places. 

 These bands have all the appearance of ordinary river gravels, and 

 prove that at certain times during the period of volcanic activity sub- 

 aerial denudation was acting upon the lava flows already solidified. 



The lavas and associated rocks cover a very large area in Jodhpur 

 territory, extending from a little east of the meridian of Jodhpur to the 

 edge of the desert between Barmer and Sind, or 145 miles from east 



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