Yol. 57.] MOUNT GIKNAB, JUNAGARH. 45 



In some of the circular colourless areas seen in microscope- 

 sections a considerable, irregularly-bounded portion, or even the 

 whole, is broken up into cloudj^ ill-defined patches with interference- 

 colours varying from white to red of the first order. The crystals 

 are usually cloudy, from the presence of innumerable minute, often 

 feathery inclusions. These are arranged so as to give the crystals 

 a striated appearance parallel to the direction of extinction, which 

 is that of the vibrations with least velocity of transmission. The 

 mineral appears to be either hydronepheline or natrolite. 



The isotropic groundmass and its alteration-products, as well as 

 the nepheJine, contain large numbers of inclusions. Even the 

 felspars, the brown hornblende, and the augite are not free from 

 them. The majority are acicular, but rounded or irregular forms are 

 very numerous in most of the spherical spaces. . All are colourless ^ 

 or cloudy, except those that are too small to show anything 

 beyond a hairlike needle or a dot. The larger acicular crystals 

 are rarely, if ever, found in the spheres, but minute needles 

 are sometimes seen. The less well-defined inclusions occur in the 

 colourless mjnerals throughout the rock. 



Most, if not all, of the inclusions in the hornblende and augite 

 appear to consist of apatite, which can also be recognized in the 

 colourless groundmass — outside the spheres — by its hexagonal 

 cross-section, high refractive index, and negative double-refraction. 

 The crystals are, however, so small that their action on polarized light 

 can only be detected (even when they are embedded in the isotropic 

 matrix) in the case of a few of the larger individuals, which happen 

 to lie with their axis parallel to the surface of the microscope- 

 section. These needles of apatite have no definite orientation, and 

 penetrate their hosts in all directions — irrespective of the structure 

 of the latter. In some cases they seem to radiate from the 

 hornblende. 



There are other acicular inclusions which present many points of 

 similarity to the apatite, but appear to have a rather lower refractive 

 index, though higher than that of the colourless material in which they 

 are embedded. The cross-section is often rhombic, yet in most cases 

 it is too small for the shape to be determined. These inclusions 

 rarely show signs of double-refraction ; this may, however, be due 

 to their small size. In most cases they follow definite directions, 

 one or more systems of parallelism being generally visible. They 

 sometimes swell out in an irregular manner, and may anastomose 

 together. Occasionally a number of parallel needles are seen to 

 project, like the teeth of a comb, from a main stem of similar 

 material. 



In the nepheline most of the inclusions lie parallel to the inter- 

 section of the base and prism, while others are parallel to the vertical 

 axis. 



^ In some cases a slight trace of a greenish-brown tint is apparent ; but this 

 is due, I believe, to reflection from adjoining hornblendes. Similar acicular 

 colourless inclusions are described as occurring in the Lake Chainplain rocks 

 [13] p. 36. 



