BY W. N. BENSON. 145 



ellipsoidal masses in pillow-lava. Within the micro-ellipsoids are 

 phenocrysts of more or less epidotised plagioclase and fresh augite 

 (Plate xxvi., fig. 10). 



Another type of structure occurs in the rapidly chilled rocks, 

 particularly in the fragments of spilite in the agglomerates asso- 

 ciated with the limestones. This has been already described, and 

 is illustrated in a previous paper (Partiii.,p.665, and Plate xxvii., 



fig.2). 



Occui'rences of various types of dolerite and sjnlite. — The two 

 widest masses of dolerite are those on Munro's Creek and Hang- 

 ing Rock. The first is about five hundred yards wide. Its 

 eastern margin, against a fine-grained tuff, is a very fine-grained 

 mass of uralite and chlorite, with a few phenocrysts (1041). 

 A yard from the margin, the grainsize is larger (0-2 to 0-3 mm.), 

 though the rock is still considerably decomposed. From this 

 there is a gradual increase in grainsize inwards, and also an 

 increase in the amount of quartz. A wide zone occurs lying 

 from one hundred to one hundred and fifty yards from the bound- 

 ary marked by a gneissic (iluxional) structure. The inner quartz- 

 dolerite has the composition given on p. 139 (1021), while the 

 rock outside the gneissic band is free from quartz and has a lower 

 percentage of silica (1002). A narrow fine-grained margin 

 appears also on the western side of the mass, and is separated 

 from the invaded rock (tuff-breccia) by a few inches of a finely 

 granular aplitic rock which merges into the tufi'. Under the 

 microscope(1182) it is seen to be made up of small (0*05 mm.), 

 equidimensional grains of albite, frequently free from twinning; 

 at other times so finely laminated with albite and pericline 

 twinning as to appear like microcline, save that its optical char- 

 acter is positive. A few small crystals of augite, and a little 

 ilmenite are also present. 



The large mass of Hanging Rock, which is half a mile wide, has 

 an acid central portion. The rock figured on Plate xxv., fig.l 

 (1065), from the central portion of the massif, is much richer in 

 silica than that near the margin, which contains 54% SiO„ (see 

 analysis 145). The grainsize does not vary noticeably across the 

 massif. 



