GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 500 



it may be noticed that they compare somewhat better with 

 the pallio-essexite (No. XII) than with the Prospect essex- 

 ites. In them the augite is to a large extent replaced by 

 a somewhat alkaline hornblende. 



Another group of rocks to which the Prospect pallio- 

 essexite is closely related in chemical and mineral compo- 

 sition is the subalkaline basalts — those which, though 

 themselves containing little or no lenad mineral, occur in 

 districts where basic lavas containing lenads are common. 

 In this group must be placed the basaltic rocks of the dyke& 

 and necks of the Sydney district in which Prospect lies, as 

 a few of them contain nepheline, 1 and the analyses of two 

 of them here quoted (No. X from Bondi and No. XII from 

 Camden Park) agree very closely wit!) that of the Prospect 

 pallio-essexite (No. XI) and essexites (Nos. V and VI). This 

 similarity of composition may be accepted as an indication 

 of their origin from a common magma. In thin section under 

 the microscope these basalts are practically indistinguish- 

 able from the Prospect pallio-essexite. Analyses of two 

 other rocks of this class are quoted in columns XIII and 

 XIV. The former is a basalt from near Clot in the north- 

 east of Spain forming part of the Western Mediterranean 

 comagmatic region recently described by Dr. Washington, 

 which includes many nepheline-basalts. It consists of 

 labradorite, a colorless augite, olivine, titaniferous magetite 

 and a little orthoclase. Hornblende and biotite are absent. 

 The latter (No. XIV) is a basalt from Uvalde County, Texas r 

 a district where basanites and nepheline- and melilite- 

 basalts are found. It contains labradorite, a very little 

 alkali-felspar (?), augite, olivine, biotite, magnetite^ and 

 apatite. 



In concluding this brief account of other rocks to which 

 the Prospect essexite closely approximates in composition,. 



1 G. W. Card, Eec. Geol. Surv. N.S.W., Vol. vn, part 2, p. 236. 

 G 1— Dec. 6, 1911. 



