352 C. A. COTTON 



junction plane, at the point examined, strikes N. 25° E., and dips 

 at an angle of 47° to the west-northwest. The irregular bedding in 

 the conglomerate is roughly parallel to the junction and apparently 

 the junction is parallel to the bedding of the Gray Marl. In the 

 latter, however, the only indication of stratification is given by 

 discontinuous bands of concretions. 



The upper layer of the Grey Marl is, lithologically, very similar 

 to the Grey Marl as a whole except that it contains some broken 

 shells and a few small pebbles or perhaps concretions of material 

 similar to the Grey Marl but more indurated and containing shells 

 in a bad state of preservation. These resemble larger masses, found 

 close at hand, at a somewhat lower horizon, but still near the top of 

 the Grey Marl series, which contain well-preserved fossils.^ It is 

 not quite clear whether they are derived bowlders or concretions. 

 Thomson regards them as the former and considers that they 

 indicate a certain amount of contemporaneous erosion. 



Immediately overlying the upper layer of Grey Marl is a layer, 

 2 ins. in thickness, of conglomerate formed of various-sized rolled 

 pebbles of graywacke (from the pre-Cretaceous formations). 

 Next follows 2 ft. 6 ins. of bedded- sandstone, covered by i ft. of 

 mudstone, and that again is followed by many feet of fairly coarse 

 conglomerate interbedded with sandstone and mudstone bands i ft. 

 to 3 ft. in thickness, and with bands of very coarse conglomerate. 



A distant view of the junction in the Mead gorge gives a false 

 appearance of unconformity with discordance of dip, and led 

 McKay to make the statement: "The overlying conglomerates are 

 quite unconformable."^ 



This appearance of unconformity is due mainly to two causes: 



I. Owing to the fact that the Grey Marl is a weak stratum, 

 while both the underlying Amuri Limestone and the overlying 

 conglomerate are very resistant, the valley of the Mead River 

 is contracted into narrow, vertical-walled gorges where it crosses 

 the conglomerate and hmestone, but, between the two gorges, it 

 opens out, with broadly flaring sides and a floor of considerable 

 breadth, to form a nearly circular hollow on the outcrop of the 



' J. Allan Thomson, N.Z. Geol. Siirv., -jth Ann. Rep., p. 123, 1913. 

 ' A. McKay, op. cit. {1886), p. 95 and section p. 94. 



