392 Transactions. — Geology. 



is now a portion of the Horseshoe Estate situated near 

 Clarendon Eailway-station, a place thirty miles south of 

 Dunedin. Subsequent search has shown that phosphatic 

 rock occurs in many places on the estate and adjoining pro- 

 perties, including, among other places of interest, the well- 

 known lime-quarries at Millburn. 



In June of this year I was enabled through the courtesy 

 of the owners to make an examination of the geological con- 

 ditions under which the phosphate occurred. The result of 

 my investigation was contained in an article contributed to 

 the Otago Witness in July* of this year. After an interval 

 of five months I again visited the locality, with the view of 

 collecting any fresh facts disclosed by the extensive mining 

 operations undertaken during that period. The result of my 

 observations on both occasions is contained in the present 

 paper. 



Physical Features. 



From the low-lying valley at the south end of Waihola 

 Lake, along which the Dunedin-Invercargill Eailway runs to 

 Milton, the land rises to the westward by a succession of low, 

 gentle, undulating hills, from which, by a long easy slope, is 

 reached the summit of the semicircular ridge whose contours 

 probably suggested the present name of the estate. The point 

 of the horseshoe is directed toward Milton, with the Millburn 

 quarries on the outer rim and Horseshoe Bush on the inner. 

 The summit of the ridge is fairly flat. The descent into the 

 bend is long and easy, but on the Milton side it is generally 

 abrupt, and in many places quite precipitous. 



The surface is open agricultural land, much of which is at 

 present under cultivation. 



General Geological Structure. 



The general geological structure of the district is very 

 well seen in the section running from Clarendon westward 

 across the Horseshoe Estate, for a distance of perhaps a mile 

 and a half from the railway-line. After leaving the flat the 

 low hills first crossed are composed of mica-schist of probably 

 Silurian age, lying nearly horizontal. Proceeding westward, 

 the schist is overlain by Upper Eocene quartz grits and 

 conglomerates which usually form the lowest member of the 

 coal-measures in southern Otago. The grits are in turn 

 followed conformably by glauconitic greensands, limestones, 

 often glauconitic, and a soft brown sandstone. The latter 

 is overlain by a flow of basalt which caps the horseshoe 

 ridge referred to above. The series of beds associated with 

 the limestone lies nearly horizontal in the Horseshoe Bend, 



* Otago Witness, 16th July, 1902. 



