114 



G. D. OSBORNE. 



trict, this neck has been known to Sydney geologists for 

 many years. On the "Geological Sketch Map of the 

 Country in the Vicinity of Sydney," issued by Mr. Pittman 

 in 1904, it is shown, but incompletely as regards extent. 

 This portion of the map, was taken from a sketch map by 

 Mr. R. N. Dart, B.E., who, visiting the neck in 1903, was 

 only able, at the time, to map portion of it. 1 



The Basin neck has been referred to several times in 

 geological literature, namely, by Benson, 2 Mawson and 

 Taylor, 3 and Woolnough/ Also the record of a chemical 

 analysis of a diallage rock from the Basin, made by Mr. 

 J. C. H. Mingaye, is given in a Mines Department publi- 

 cation. 5 



The chief interest attaching to the Basin neck is the 

 occurrence of xenoliths, mostly of igneous rock. These 

 are included in both breccia and basalt, chiefly in the latter. 

 In the basalt only cognate xenoliths occur. 



This and similar occurrences in New South Wales of 

 basic and ultrabasic xenoliths in generally less basic igneous 

 rocks are matters of more than passing petrographical 

 interest. Some have been treated in more or less detail 

 by Profs. David and Benson, Mr. Sussmilch and others. 6 



As far as we know all our examples of Tertiary ultrabasic 

 rocks occur as xenoliths. The mass of picrite within the 

 dolerite in Jellore Creek, Mittagong, seems from the details 

 given in the paper by Taylor and Mawson 7 to be an example, 



1 Verbal communication from Mr. Dart. 



2 (a) Journ. Boy. Soc. N.S.W., Vol. 44, p. 548. (6) P.L.S., N.S.W., 

 1914, p. 452-3. 



3 Jonrn. Eoy. Soc. N.S.W., Vol. 37, p. 349. 



4 "N. S. Wales Historical, Physiographical and Economics," p. 92. 



5 Ann. Eept. Mines Dept., N.S.W., 1908, p. 174. 



6 For detail of references see Benson, Proc. Roy. Soc. N.S.W., Vol. 44, 

 pp. 496, 548. 



7 Journ. Eoy. Soc. N.S.W., 1903, p. 326, and fig. 8. 



