468 PROF. W. S. BOTJLTON ON A MONCHIQTJITE INTRUSION [Nov. I9H, 



A close search has failed to reveal any undoubted nepheline in 

 this ground-mass, but a fibrous, colourless, faintly-birefringent 

 zeolite, generally occurring in radial aggregates, and possessing a 

 low refraction, is probably the mineral natrolite, and may 

 possibly result from the decomposition of nepheline. 



Ocellar spaces are frequently present, some quite small, 

 circular or elongated, with a parallel arrangement, doubtless repre- 

 senting original vesicles. Others are much bigger and more 

 irregular, suggestive of drusy cavities. They are now filled with 

 pale-green chloritic matter; carbonates, often in beautifully 

 sharp rhombs with marked zonal structure ; and perfectly clear 

 and colourless, idiomorphic or sub-idiomorphic and isotropic crystals 

 of secondary 1 analcite (fig. 3B, p. 467). 



Very minute, colourless or yellowish needles in feathery bunches 

 can also be made out, which usually polarize in bright tints. 

 These have proved too small for exact determination ; they may be 

 muscovite or a mineral of the zeolite group. 



Many of the spaces show a zonal arrangement, most frequently 

 with the pale-green chloritic substance occupying the centre of the 

 space and enclosing the carbonate rhombs ; while the analcite- 

 crystals and the fibrous substance line the walls of the cavity 

 (fig.3E). 



Some, again, have very indefinite outlines. They are merely 

 clearer spaces merging gradually into the surrounding ground- 

 mass. In such cases the clear space is occupied by allotriomorphic 

 analcite, and perhaps some carbonate and chlorite, while small 

 biotite-needles and augite-prisms with octahedra of magnetite range 

 themselves peripherally, projecting some distance into the clear 

 space (fig. 3 A). 



The small ocelli pass by gradual stages into the larger pale pink 

 and white patches so noticeable in the hand-specimen, including 

 those which have banded agate-like or mammillated structure, and 

 where the infilling material is often iron-stained chalcedonic silica. 



Some of the rock-sections reveal special points of interest. 

 Slide M 401 may be taken as an example (PI. XXXVII, fig. 4). 

 It shows a small xenolith of fine-grained sandstone about half an 

 inch across, and surrounding it are numerous glassy fragments of 

 the igneous rock, showing a striking resemblance to palagonite-tuff. 

 The lapilli consist of the usual brown or yellowish-red basic glass, 

 enclosing small, perfectly spherical vesicles, now occupied by zeolites ; 

 while minute prisms of augite, olivine, and grains of quartz are also 

 embedded in this glass. These fragments are set in a pale-grey or 

 colourless matrix made up of abundant rhombs of carbonate, 

 zeolites, and small fragments of sandstone. 



The glassy character of these lapilli is doubtless to be explained 



1 The term secondary is here used without implying any considerable 

 interval between the formation of the analcite of the ground-mass and that of 

 the ocelli. 



