500 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOK AND C. A. SUSSMILCH. 



On the other hand there are certain appearances which 

 at first sight suggest that some of the analcite is primary. 

 In many sections analcite crystals are found mostly free 

 from apatite inclusions and presenting idiomorphic outlines 

 to masses of stilbite or chlorite (see Plate XXXIX, fig. 4). 

 Closer inspection shows, however, that crystal faces are 

 commonly absent on one side; and there is no doubt that 

 these crystals are a late product of alteration, being 

 deposited in cavities which were subsequently filled up by 

 another mineral. This view is supported by the fact that 

 beautiful hand specimens of analcite in eikositetrahedra 

 may be obtained from geodes in parts of the Prospect rock 

 which are much decomposed. Whether the cavities are 

 miarolitic (i.e. irregular steam holes), or whether they have 

 been formed by the complete removal of olivine, felspar, or 

 augite, by the percolating waters, leaving a space which 

 was subsequently filled, there is no means of certainly 

 deciding. We see nothing to preclude the latter mode of 

 formation, but cannot prove its actual operation. 



In one instance, observed in slide T, (Specimen 2154 

 Sydney University Collection), analcite was noticed which 

 presented apparently idiomorphic outlines to perfectly fresh 

 felspar and augite; it contains inclusions of apatite, and 

 hence was not formed in a cavity. This case is probably 

 to be explained as a phenomenon due to the accidental 

 conjunction of faces of augite and felspar in such a way as 

 to give a section of the analcite crystal with false faces 

 inclined to one another at about the same angle as the 

 faces would give in a section of the eikositetrahedron. In 

 the pallio-essexite are occasionally found rounded masses 

 of analcite as described in specimen L. These could per- 

 haps be interpreted as phenocrysts of analcite partially 

 resorbed; but their want of uniformity in size is against 

 this view, and they are easily explained as spherical vesicles 

 filled with secondary analcite. 



