164: Transactions. 



Morgan (1916, p. 14) advocates a Cretaceous age for the coal of Brighton 

 and Green Island. 



Unfortunately, McKay's fossils from Green Island do not settle the 

 question as to whether the coals of the whole area are of the same age, for 

 doubt might well be expressed whether the shelly limestone of his collection 

 really came from Green Island. In his report there is no mention of this 

 rock, but only of the concretionary greensand. All the specimens show 

 some weathering on one side, as if they had been collected from an out- 

 crop. This would indicate that he did not separate the fossils obtained 

 from the greensand from those obtained at Brighton. On the other hand, 

 it is difficult to think such a mistake has been made, when it is known that 

 McKay himself drew up the list of fossils for Green Island and included in 

 it the belemnite. The weathering observed on the limestone could have 

 been produced while the specimens were exposed on the surface a few years. 

 Hector's Progress Report (1877, p. xiii) on McKay's journey reads as if 

 the belemnite was collected from Green Island. The uncertainty may be 

 dispelled if Christies sink a shaft in the neighbourhood of Walton Park. 



Other considerations will show that the coal-measures of Green Island 

 and Brighton are of the same age. There is a lithological similarity 

 between the coal-measures and quartz sands of the two places. The main 

 seam, containing the best coal, is the second seam from the base. Also, 

 the quartz sands are everywhere about 50 ft. thick, and have a band of 

 conglomerate or gravel in the uppermost portion. The sequence as inter- 

 preted by Park finds no parallel in Otago. If the coals were of different 

 ages the Brighton beds would be limited to that locality, for the shaft put 

 down in th.e Saddle Hill Colliery workings did not encounter another set 

 of coal-bearing beds. 



Glauconitic Mudstone. 



This formation, which is 800 ft. thick, has a fairly wide extent. It forms 

 the main part of the valley of Abbott's and Waterfall Creeks, the hill above 

 Loudon's mine, and the land around Stony Hill and above Brighton. 



The dip, determined from the outcrop as a whole and single exposures, 

 is found to be the same as that of the underlying coal. The basal part of the 

 formation is more sandy than the average mudstone. High up in the beds — 

 e.g . by Saddle Hill and in Abbott's Creek — there is a close resemblance to 

 a marl. In Waterfall Creek, 7 chains below the waterfall, the uppermost 

 portion is represented by the following : Cemented glauconitic sandstone, 

 1 ft. 6 in. ; micaceous marl, 9 ft. ; cemented glauconitic sandstone, 2 ft. ; 

 glauconitic sandstone, 80 ft. 



Thin sections of the cemented greensand show angular quartz-grains 

 0-4 mm. by 0-4 mm., muscovite, green glauconite exhibiting pleochroism, 

 and oligoclase twinned on the albite law. The position and shape of some 

 of the grains of glauconite indicate that the mineral was deposited in the 

 chambers of Foraminifera, and that subsequently the shell was dissolved 

 away (see Plate XXXVI, fig. 1). The oligoclase could not have been derived 

 from the schist, because the latter has its plagioclase feldspar twinned on 

 Carlsbad but never on the albite law. 



On the under-surface of a projecting ledge of cemented glauconite sand- 

 stone in Waterfall Creek crystals of gypsum about 2-5 mm. in length were 

 obtained. They can be recognized by the pinacoidal cleavage (010) and 

 the typical minus-unit pyramids (111). Sulphuric acid, which is produced 

 during the weathering of the iron sulphide contained in the sandstone, has 

 reacted on the calcite of the shells to form calcium sulphate. 



