364 GREAT SERPENTINE BELT OF NEW SOUTH WALES, vii., 



phyre. The conclusions previously reached, as to the mode of 

 origin of mag'netite-keratophyres, and their relation to normal 

 keratophyres, and jaspers are quite confirmed. 



Dolerites^ and Sjnlifes of the Eastern Series and the Transitional 



Zone, 

 In Portion 43, tliere occurs a very quartzose dolerite, No. 1398. 

 The felspar occurs in perfect prisms and prismoids. It is an ande- 

 sine only slightly zoned and very turbid. The augite is in idiomor- 

 phic grains, with a normal optic axial angle. It is more or less 

 replaced by chlorite. Ilmenite forms large angular grains; and 

 quartz is very abundant in large interstitial grains, and is never 

 granophyric. The spilite (1390) which occurs in Portion 36 hy 

 tlie western boundary of Portion 47, is a very fine, even-grained 



* In a review of the earlier parts of tliis series of papers, Professor 

 Johannsen (Journ, of Geol., 1917, p,494) writes: — "The term dolerite 

 is apparently used in a diflerent sense from that common in the United 

 States, where it signifies a coarse-grained basalt containing a basic plagio- 

 clase. The writer speaks of albitization proceeding inwards in the felspar, 

 by which he apparently means that the sodic rims are secondary. It would 

 seem more probable that the zonal rims are primary. The rock thus ap- 

 pears to be an augite-andesite." It is, therefore, desirable to state that 

 the albitic rocks indicated are structurally, and in their mode of occur- 

 rence, similar to those termed albite-diabases by Flett and Dewey (Geol. 

 Mag., 1911), and differ from them chiefly in the better preservation of the 

 pjTOxene. Subsequent research has shown that they are associated with 

 rocks with basic plagioclase, which are quite similar to, sa3% the Devonian 

 diabases of Germany. The name dolerite was adopted in conformity with 

 Dr. Marker's practice (see Petrology for Students, 4th Edition) to indicate 

 the original nature of the rock. The term augite-andesite might have been 

 misleading. The secondary nature of the albite was inferred partly from 

 comparison with the analogous rocks studied by Flett and Dewey. Sub- 

 sequent work led the writer to doubt the universality of the albitisation, 

 and the present investigation leaves him with the feeling that the albite 

 may sometimes be a primary magmatic crystallisation, sometimes the 

 result of secondary post-volcanic replacement bj" magmatic solutions, 

 Analj'ses show that such albitisation is accompanied by an addition of 

 soda. In the case of analcite, it has been shown that these two stages 

 merge into one another, and it is suggested that the same may hold in 

 regard to albite, 



