o*G LA TOUCHE: GEOLOGY OF WESTERN RAJPUTANA. 



That they were formed, or in process of formation, while the surround- 

 ing magma was in a more or less viscous condition is proved by the 

 manner in which this " flows " round them ; and their dimensions pre- 

 clude the idea of their having been originally cavities, which have been 

 subsequently filled with minerals leached out from the surrounding 

 rock, for they are sometimes several inches long and half an inch or 

 more in width. If they had been cavities, we should expect to find 

 spaces in the centre of them unfilled with mineral, and I have so far not 

 found anything of the kind. True cavities filled with chalcedony or 

 other secondary mineral do occur sometimes, but they present an 

 appearance very different from the lenticular bands in the fluidal rocks. 



Some few of the specimens I have examined exhibit a very well 

 defined perlitic structure. This is especially well developed in No. 

 1 1 "494 (PI. XI, figs. 1,2), from a flow in the neighbourhood of Jodhpur, 

 and is associated with minute sphaerulites. No. 1 1'530 also shows this 

 structure very clearly. In No. 1 1-532 (PI. X, fig. 5) the perlitic cracks 

 are concentric with the quartz phenocrysts and sometimes extend into 

 them from the ground mass. The perlitic structure in all of these is of 

 the ordinary kind, but the beautiful variety shown in portions of 

 No. 11-523 (PI. X, fig. 6), merits a further description. In this case 

 the cracks have opened to a small extent, and are lined on either side 

 by minute crystals of quartz, projecting inwards, and meeting along a 

 well defined line in the middle of the fissure. The spaces between the 

 cracks are filled with granular quartz, sometimes clear, but usually 

 stained a deep brown. This structure is identical, so far as can be 

 determined from the description, with that described by Rosenbusch 

 in the pyromeride of Wuenheim in Upper Alsace. 1 This specimen, which 

 was collected in the hills to the west of Nagona, about 33 miles west- 

 south-west of Jodhpur, also contains large brown sphaerulites, some of 

 them as large as a hazelnut. 



Sphserulitic structures are of very common occurrence in these rocks 



Rosenbusch. Mikr. Pliys. Massig. Gest., p. 83. 

 ( So ) 



