BY W. X. BEXSONj W. S. DUN, AND W. It. BROWNE. 413 



I. Trachytoid quartz keratojiliyre, Clmrch Hill, (JuiTabubiihi. Analyst, H. 



Yates. 

 II.* Kliyolitt', Aiu'on Hill, Panama Canal Zone. Analyst, G. Steiger. 

 lll.f (|)iiai'tz keratopliyre, Portion 175, Par. Neminglia, N.S.W. Analyst, W. N. 



Benson . 

 IV. t Soda rbyolite, Paddy's Sngarloaf, nr. Raymond Terrace, N.S.W. Analyst, 

 W.'a. Grieg.' 



The analysis indicates the sodic character of the rock, although the CaO per- 

 centage is rather high. It also points to the presence of about 10 % of ortho- 

 olase, either as a definite mineral or in solid solution in ther oligoclase. The cor- 

 respondence of this analysis with that of the rbyolite from the Panama Canal 

 Zone is very striking. Analyses 111. and IV. have been inserted for contrast 

 rather than comparison; there are evidently important differences between the 

 Carboniferous rock and the Devonian quartz keratophyre from Nemingha, while 

 the Paddy's Sugarloaf rock, which is probably to be correlated, chemically as well 

 as petrologically, with the soda rhyolites of Rocky Creek, emphasises the difference 

 between these rocks and the trachytoid quartz keratophyres . 



This Clhurch Hill intrusion is an offshoot from or prolongation of a laccolitic 

 mass invading- the Werrie basalt and forming a small hill about a mile west of 

 Currabubula. The rock composing the intrusion differs but little from that just 

 described, but shows some additional features of interest. The cavities in the 

 rock have not always been filled completely with quartz, and open spaces may be 

 left, into which project felspar crystals. These spaces may, however, be filled 

 with calcite or more frequently with what appears to be a zeolite, having a R.I. 

 of about 1.49, complex twinning, and a birefringence about the same as that of 

 quartz. This mineral has evidently formed after the consolidation of the quartz, 

 which it occasionally enwraps. Embedded in both quartz and zeolite are isolated 

 little crystal aggregates of felspar and an occasional flake of rather pale-coloured 

 biotite. The conclusion seems warranted that the zeolite represents the last 

 and the quartz the penultimate stage in the continuous process of rock-crystalliza- 

 tion . 



The rock (1520, 1526) composing the laccolitic mass S.W. of Soma has some 

 features which mark it off from the foregoing types. It is very fine-grained, with 

 a few small plienocrysts of oligoclase, and a gToundma.ss composed mainly of little 

 albite laths and quartz, with ])robably some orthoelase. In contrast to its usual 

 habit, the quartz is characterised by distinctly squarish outlines, sometimes with cor- 

 rosion embaymeuts, as though it was of early crystallization. It makes up about 

 40 % of the slide . Bleached and ragged biotite and a little ilmenite are seen . 



An interesting rock is that found in a kind of sill cutting through a mass of 

 felsite on Werrie's Creek. The plienocrysts include, in addition to oligoclase and 

 (?) orthoelase. infrequent chloritized crystals of augite and hypersthene. The 

 gi'oundmass is of stumpy oligoclase with some orthoelase, abundan.t magnetite, a 

 little pyrites and apatite, and a good deal of quartz and biotite. 



The presence of both rhombic and monoclinie pyroxene links up this rock 

 with the andesites. 



•Washingtou's Tables (United States Geol. Surv., Prof. Paper 99), p. 231. 

 tThese Proceedings, xliii., 1918, p. 602. 

 ^Washington's Tables, p.lf).3. 



