Physiography oj Bulla Area. 341 



.conclusion is upheld by the presence of a conglomerate in the 

 .southern exposure. 



No evidence of stream action in the two patches to the north was 

 found, but this may have been hidden by the talus, which masks 

 -the surface. The northern outcrops are at a lower level than the 

 soutliern one, and this suggests that the stream in this locality 

 flowed to the N.N.E. 

 \ 

 Basic Hypahyssal Rocks. 



Along Jackson's Creek, in the south of the area, there occur at 

 least seven basic to ultrabasic dykes. D^ between D, and D.^ can 

 be seen only when the creek is very low. In places it is sill-like. 

 T)j is 22 ft. wide, and is found directly beneath the " Organ 

 Pipes," but sejiarated from them by Kainozoic grits. D^ contains 

 •nodules of relatively fresh material, the only unaltered mineral 

 being ajDatite, though abundant augite and olivine can be recog- 

 nised by the shape of the old crystals and their alteration pi'odvicts. 

 «Oblong outlines which may have been plagioclase laths are fairly 

 numerous. The apatite has a vitreous to subresinous lustre. It.s 

 shape is remarkable, for it resembles a miniature torpedo. Fre- 

 quently the crystals are long, but flattened. They can be obtained 

 up to 1;!^ in. in length, and down to the smallest needles. All have 

 smooth rounded outlines, but some have small smooth depressions. 

 The smooth outlines and depressions are evidently due to corrosion 

 by the magma. Owing to the extreme brittleness of these crystals, 

 it is difficult to obtain complete specimens. From carefully chosen 

 fragments the S.G. was found to be 3.104, and the P-oO^ content 

 40.3 %. A brisk effervescence is set up on dissolving the mineral 

 in hot HCl. 



Phenocrysta of olivine, augite, apatite, and felspar were set in a 

 fine groundmass. Of these apatite is the only survivor. 



The seven dykes appear to be of the same age, and of the same 

 material, though apatite was found only in D^. All have the same 

 brownish yellow appearance, the same greasy feeling, the same 

 <legree of decomposition, and the same rich iron content, as shown 

 "by the iron oxide on the footwalls. 



Age of the Dykes. — From their field appearance the basic dykes 

 •are all of the same age. The fact that the dykes (D^and D^) are 

 not intrusive into the overlying stratified Kainozoic grits, shows 

 that they are older than either the NeAver Basalt or the Kainozoic 

 grits. The fact that they are intrusive into the Silurian sedi- 

 ments, and were not affected bv the folding agencies of Lr. Devo- 



