56 G. T. Trechmann — Age of the 



Maitai Beds, as remarked above, form another syncline. Both these 

 parallel synclines appear to have been faulted and thrust over towards 

 the west or northwest, with the result that the Trias appears to dip 

 below the Maitai Limestone. Some zones of the Trias are locally 

 missing through being faulted or thrust out, and the Tertiary beds 

 which bound the Trias to the north-west along the edge of the 

 Waimea Plains are forced up on end along the junction. 



Lithology. 



The Maitai Limestone in the Wairoa Gorge is much jointed, fissured, 

 slickensided, and crushed, and has many veins running through it, 

 which are sometimes seen to be repeatedly faulted on a small scale. 

 Some parts are very impure, containing quartz grains and spots of 

 a serpentinous or glauconitic mineral in such quantity that very little 

 ealcite remains and the rock becomes decalcified to a considerable 

 depth. It is splintery in places but very hard in others. The 

 greater part is unfossiliferous; the fossils appear to be confined to 

 the upper part, where it adjoins the Maitai slates and greywackes 

 to the east. Little clusters of prismatic shell fragments, however, 

 occur lower down. The fossils are very closely incorporated with the 

 rock, and are in consequence difficult to detach and are best collected 

 in the decalcified portions. 1 had a slice made of a portion of the 

 rock which contained a cluster of prismatic shell fragments (Plate IV, 

 Fig. 8). It contains many fragments with corroded edges, and the 

 surrounding matrix is also largely made up of isolated prisms of the 

 decomposed shell which seem to be arranged round the larger pieces 

 in a sort of flow-structure, or this may be a later pressure effect. A cross 

 section of the prisms shows that they are more or less octagonal in 

 outline. The matrix also contains quartz grains and chloritic specks. 



The thickness of the limestone in the Wairoa Gorge is estimated at 

 2,000 feet, 1 but at Wooded Peak it is reduced to about 1,000, while 

 southwards it gradually thins out and is finally represented by 

 calcareous slates. It dips everywhere very steeply, often vertically. 

 It seems impossible at present to estimate with any accuracy the 

 thickness of the slates and argillites and the other beds which 

 form the Maitai Series, but it is undoubtedly very great. The beds at 

 Wooded Peak form a great series of hard, thinly bedded, well-jointed 

 grey argillites, which dip everywhere nearly vertically. The large 

 flattened shells are difficult to see and are easily overlooked on the 

 weathered and moss-grown surfaces of the rock, but the sections of 

 them can be detected on examining the joint faces, and the application 

 of a hammer and chisel soon brings them to light. They are chiefly 

 located along one or two bands quite near the adjoining limestone. 



Fossils of the Maitai B,ocks. 



Aphanaia sp. (PI. IV, Figs. 1-4.) 



A glance at Figs. 1, 3, and 4 will show that the hinge of the large 



bivalve of Wooded Peak is not that of hioceramus, but the genus of 



this shell is not easy to determine beyond doubt. I am of the 



1 McKay, " Beport on the Wairoa and Dun Mt. Districts " : Rep. Geol. Expl. 

 for 1877-8, issued 1878, p. 135. 



