' BY W. H. TWELVETREES AND W. P. PETTERD. 4^)' 



of occuiTence. Mr. R. M. Johnston places the geological 

 horizon of this rock at the close of the Trias- Jura system, 

 and we have no doubt in this his opinion is approximately 

 correct. The rock has never been found in situ in any of 

 our Tertiary beds, which, however, do contain included 

 fragments of it. The microscopical appearances of speci- 

 mens from widely distant parts of the island also support 

 the inference that the rock all over the Colony belongs to 

 one and the same geological age. In one instance we have 

 noticed microscopical fragments of the dolerite (diabase) 

 included in Tertiary oli vine-basalt. This occurs near 

 Bothwell, where the basalt probably has entangled in its 

 flow loose pieces of the older rock. 



Its mineralogical constitution is ratlier simple, as will be 

 seen from the following list of constituents : — 



Essential. Accessory. Secondary. 



Plagioclase. Olivine. Chlorite. 



Augite. Apatite. Serpentine. 



llmenite. Actinolite. 



Magnetite, Scolecite. 



Pyrite. Calcite. 



Mica. 



Quartz. 



Oligoclase. (?) 



Microscopical Characters. 

 Plag ioclase-FeUpa r. — The sections are lath-shaped, in 

 short or long laths, sometimes in tabular forms. Of course, 

 it must be remembered that these sections only give us a 

 view of one particular plane, and convey to the mind, 

 merely inferentially, the image of the solid crystal. The 

 felspars could not be called lath-shaped ; lath-shaped 

 sections are all that is meant. Out of so many sections it 

 is surprising to find how few are available for measure- 

 ments of the extinction angle. For this purpose only such 

 twins can ))e selected as give approximately symmetrical 

 extinctions on opposite sides of the trace of composition 

 plane. The twin forms are Carlsbad and Albite. We 

 have seen none on the pericline type. If we take the 

 haphazard sections of felspars in this rock, we shall find 

 some giving low, some high, angles. The low-angled ones 

 are probably those parallel to the base ; the high-angled 

 ones parallel to OK), and the maxima of extinctions are 

 given by the latter. The highest angle we have noticed i& 

 42°, but, as a rule, angles of 30°, 32° are obtained. From 

 this the felspar may be inferred to be labradorite and lab- 

 radorite-anorthite. 



