102 GEOLOGY OF THE SON VALLEY, ETC. 



quartz ; but the next rock from the same exposure T ^ shows it in abuncU 

 ance. In this rock the porphyritic constituents are plagioclase, or- 

 thoclase, quartz, and biotite. The quartz is corroded to such an extent 

 by the magma that all crystal outline is obliterated. The felspars are 

 quite idiomorphic, although they have been also slightly corroded, 

 particularly the orthoclases. The plagioclase shows beautiful zoning, 

 while the orthoclase exhibits obscurely the initial stage of the micro- 

 cline condition. As in T i^- the felspars are partly altered with pro- 

 duction of epidote, and this mineral, together with chlorite, has also 

 resulted from the alteration of the biotites. The once molten magma 

 is entirely crystallised, the structure being spherulitic, or rather 

 pseudo-spherulitic, for the spherulites consist of quartz and felspar. 

 In many rhyolites, although this composite structure of the spheru- 

 lites is very probable, yet it cannot be ascertained beyond doubt 

 on account of the minute dimension of their fibres. In the present 

 case it is indicated by the great irregularity of the dark cross which 

 they exhibit in polarised light ; and it is definitely established by 

 examining them with the highest power of the microscope, for it is 

 then observed that the structure is an exceedingly minute micro- 

 graphic one ; so minute as to be just upon the limits of visibility, yet 

 perfectly distinct. 



The groundmass of -^ is therefore comparable to that of T |^ 

 being only on a very much more minute scale, and it is probable that 

 a detailed study of the Kalapahar outcrop would bring to light a com- 

 plete series of gradations. 



Similar rocks exist in the neighbourhood of Jupla (Lat. 24 32', 



Rocks near jupla, col- Long.8 4 °4')« This is how Mr. Medlicott de- 

 lected by Mr. Medlicott. scribes the occurrence : " A more extensive patch 

 of the same rock occurs at the very edge of the crystalline area South- 

 East of Jupla, and in the little detached hill of Aliuneggur. Here 

 several varieties of granitoid and gneissose rock are clearly overlaid 

 by different forms of the derivative stratified rocks, both compact and 

 pseudo-crystalline" The collection contains a specimen of this rock 

 (No. -i^) labelled (< trappoid rock resting on the dome-gneiss of 

 ( 102 ) 



