BY H. I. JENSEN. 883 



Linking the foregoing basic rocks to the alkaline series are 

 certain remarkable essexites, of which I have found rolled 

 specimens in the creeks, but which I have not met with m situ. 



N.28 is a reddish coarse-grained rock, in handspecimen not 

 unlike N.27 (described hereafter). Loc: Thor's Creek, Bullawa 

 Creek. 



Microscopic examination : Texture holocrystalline, hypidio- 

 morphic granular, uneven-grained rock, showing ophitic 

 structure. 



Constituents (in order of decreasing abundance): (1) Titani- 

 ferous augite studded with interpenetrating felspar needles and 

 apatite inclusions; it occurs in phenocrysts which are more or 

 less corroded and fractured. (2) Bronzite with well-defined 

 crystalline outlines. (3) Ilmenite. (4) Analcite (interstitial). 

 (5) Chlorite replacing biotite. (6) Bytownite and anorthite 

 felspar in laths. (7) A little biotite. (8) Apatite needles. 

 (9) Zircon. (10) Zeolites of the mesolite group. (11) Interstitial 

 orthoclase. (12) Serpentine in irregular patches. 



Name : Ophitic Analcite Essexite allied to Teschenite. 

 Note. — This rock seems to have been formed by a kind of 

 magmatic mixture of an alkaline rock with a dolerite like N.17. 

 The mixture may have taken place either by an alkaline magma 

 having intruded, partially fused and assimilated a dolerite, or 

 wholly by pneumatolytic processes. The broken nature of the 

 pyroxene phenocrysts and the irregular serpentine patches 

 representing the remnants of resorbed olivines support the first 

 supposition. The second alternative receives support from the 

 facts that biotite has developed and analcite is abundant. 

 When we consider the basicity of the felspar laths and their 

 fresh appearance it becomes evident at the same time that the 

 analcite could not have been primary, nor can it have 

 been formed by decomposition of the felspar. Therefore 

 it is concluded that the magmatic vapours from the alkaline 

 intrusions caused a partial recrystallisation in this basic rock, 

 introducing KgO to form the interstitial orthoclase and biotite, 



