396 Transactions. — Geology. 



in many other localities which need not be specified it has 

 been designated by a local name. 



This limestone is very variable in physical character and 

 composition. Even in the same horizontal plane it may be 

 seen to pass gradually and insensibly from a compact lime- 

 stone into a calcareous sandstone, often within a distance of 

 half a mile or less. 



(a.) Brown Sandstone.- — From the upper surface of the 

 limestone to the basalt cap there is an interval of 120 ft. to 

 150 ft. in vertical height, apparently occupied by a yellowish- 

 brown sandstone, the character and disposition of which could 

 not be ascertained on account of its outcrop being obscured by 

 a heavy slope deposit of black earth mixed with sand. 



In the Oamaru and Weka Pass districts, where the 

 sequence of Lower Tertiary strata is very complete and cha- 

 racteristic, the Oamaru and Weka Pass calcareous sand- 

 stones, which, as we have seen, are the time-equivalents of 

 the Millburn limestone, are followed quite conformably by 

 the Hutchison quarry, or Mount Brown beds, which consist 

 of yellowish-brown calcareous sandstones containing a rich 

 assemblage of marine forms. This overlying series is so 

 closely associated with the Oamaru series that it cannot be 

 regarded as a separate formation, but only as the closing 

 horizon of the Oamaru series itself. Until something more 

 definite is ascertained about the sandstone lving above the 

 limestone on the Horseshoe Estate, it may be correlated with 

 the Hutchison quarry horizon of the Oamaru formation. 



Basalt. 



This occurs as a true flow. It rests on the upper surface 

 of the brown sandstone and caps all the higher hills. As its 

 junction with the underlying rock is everywhere obscured by 

 slope deposit, its thickness cannot be determined, but at the 

 old cemetery quarry the depth of the flow is not less than 

 100 ft. 



This basalt is excessively fine in texture, at most places 

 possessing a clean, splintery fracture. In the face of the 

 cemetery quarry it exhibits a rudely columnar structure. 

 Here, also, its weathered surfaces possess a deeply corroded 

 appearance, and the usual splintery character is absent except 

 in one narrow band near the centre of the higher part of the 

 quarry-face. 



In polarised light thin slices of this rock show an abundant 

 dull-grey or semi-opaque feldspathic base, with augite and 

 olivine, the latter often serpentinised. Idiomorphs of feldspar 

 are absent. The base, however, is crowded with acicular 

 microlites, some of which appear to exhibit polysynthetic 

 twinning. Magnetite is very abundant. 



