Thomson. — The Gem Gravels of Kakanui. 



487 



Head the breccia forms a small anticline, or, rather, elongated 

 dome, as can be seen by tracing the outcrop-lines exposed on the 

 beach. 



The breccia is 

 very calcareous and 

 is roughly bedded, 

 suggesting a sub- 

 marine origin. The 

 size of the included 

 minerals diminishes 

 on proceeding from 

 the axis of the 

 anticline outwards. 

 The following are 

 the largest dimen- 

 sions of the rocks 

 and minerals ex- 

 posed here (the two 

 dimensions refer to 

 the two largest in 

 cross - section ex- 

 posed on the sur- 

 face of the brec- 

 cia) : Hornblende, 

 100 mm. by 75 mm. ; 

 black augite, 115 

 mm. by 90 mm. ; 

 feldspar, 70 mm. by 

 40 mm. ; garnet, 12 mm. by 10 mm. ; green augite, 4 mm. by 

 5 mm. ; basalt inclusions, 25 mm. by 30 mm. ; holocrystalline 

 inclusions, 75 mm. by 40 mm. 



The thickness of the breccia is rather difficult to determine, 

 but at least 100 yards across the strike is exposed here, which 

 at an average dip of 26° gives a minimum thickness of 130 ft. 

 Further north, along the beach, the dip changes to the opposite 

 direction, showing evidently a synclinal curve. The " barren 

 breccia " also varies considerably in dip, and appears to form 

 an anticline. It is slightly faulted at one place, the displace- 

 ment being 18 in. to the northwards, and the hade of the fault 

 60° N. Near the fault is a fissure, filled evidently from above 

 with the materials that form the ; ' mineral breccia " ; this 

 filling, withstanding erosion a little better than the " barren 

 breccia," stands out like a dyke. Similar vertical cracks in a 

 tuff at Oamaru are filled with limestone, and at first sight seem 

 to be vertical beds of limestone. The " barren breccia " be- 

 comes flatter to the northwards in its upper layers, the dip 



IV. Sketch-plan of Outcrops of Mineral 

 Breccia on the Beach, North Head, Ka- 

 kanui, SHOWING PERICLINAL FOLD. 



