BY W. N. BENSON. 595 



to decompose. Its compositiou, determined from the Carlsbad- 

 albite twin by means of Professor Becke's diagram,* is Abg o An^ q. 

 The phenocrysts of augite are generally rather smaller, have a 

 large optic axial angle, and are decomposed on the periphery. 



Dolerites also occur with albite as their felspar, and of these 

 there are two main groups, the granitoid and the poryphyritic. 

 Rock 1120, which occurred adjacent to the rocks with basic felspars 

 in Portion 175, Nemingha, is like them in texture and grain-size. 

 The felspars are fresh and dusty, are slightly zoned, and have the 

 composition of albite-oligoelase. They are almost idiomorphic, 

 and are surrounded by a micrographic intergrowth. The pyroxenes 

 are almost completely changed to chlorite, ilmenite is abundant, and 

 quartz appears in a few separate grains, as well as in the inter- 

 growths, A few large grains of apatite occur, but these are not 

 present in the intergrowth. From its textural identity with the 

 calcic rocks of the district, it seems possible that this may have been 

 derived from such rocks by albitisation. Other albitie rocks are 

 represented by slides 1024, 1030, 1039, 1048, 117 occurring respec- 

 tively with porphyritic spilites at Tintinhull (Portion 123, Tam- 

 worth), in Portion 202, Nemingha, with spilites at Pullman's Hill, 

 Portions 205, 48, Nemingha, in Portion 204 (the northward con- 

 tinuation of the belt of calcic dolerite which invades the porphyritic 

 spilite of East Gap Hill), and in Portion 181, at the southern end 

 of West Gap Hill, associated with pyroclastic rocks. The first 

 four of these occur about half a mile from the nearest outcrop of 

 the granite, and there is no definite evidence of contact-meta- 

 morpliism. The plagioclase is clear, and very little, if at all, 

 spongy. Quartz-grains occur in small amount, but there is no 

 granophj^ic intergrowth. The ilmenite has been more or less con- 

 verted into titanomorphite. 



The porphyritic dolerites of Tintinhull have already been briefly 

 described. They are sometimes aphanitic, with small often reddish 

 phenocrysts of felspar (1138, 1160). This is a water-clear albite, 



* As yet unpublished, but in use in the laboratory at Vieiuia, during 

 the writer's visit in 1914. 



