River to Tutaekuri River is mapped as middle sub-schist. The approximate width of the belt is defined in the upper Trent River section.

Content. The rocks are more metamorphosed than the lithologically similar upper sub-schists. Sandstone, siltstone, and mudstone form the bulk. Conglomerate boulders are not uncommon in upper Trent River and have probably been derived from this belt. Volcanics and associated sediments were seen only near Trent Saddle. Red slates crop out at the saddle itself and red jaspillite on the south side of the stream that flows into Haupiri River about half a mile west of the saddle. Green tuffaceous mudstone and sandstone occur nearby.

Slaty cleavage, the most significant mappable feature of this belt, is conspicuous in all the mudstone and siltstone bands Cleavage is absent, not only from the less metamorphosed upper sub-schists to the east, but also from the more metamorphosed lower sub-schists to the west. The Trent Gorge section showed that the zone of transition from the uncleaved to the well-cleaved mudstone is narrow enough to be used as a map boundary. The incoming of cleavage cannot be accounted for by any change in the original nature of the rocks because the two belts differ little in composition. Nor is it likely to be due to increase in the degree of folding, because both belts have been strongly folded. It is suggested that the depth of burial of the rocks at the time of folding controlled cleavage. On this interpretation the uncleaved upper sub-schists were not buried deep enough to have been cleaved.

Fossil Content. The fossil Torlessia mackayi has been reported to the south (Morgan, 1908, p. 80; Cox, 1877, p. 78) in rocks of about this rank. In spite of careful search the fossil was not found. Traces of a larger fossil are common in cleaved dark mudstone in locally derived screes at one place on the south side of the Trent Gorge south-east of Mt. Monotis. Evidently at one time they consisted of masses of calcite up to three inches in length and up to a fifth of an inch thick. Most of the calcite has been weathered out and the rock is calcareous only in patches. Although quite unidentifiable, the close similarity of all the fragments, and the presence of calcite in otherwise non-calcareous rocks is evidence for an organic origin. Many of the weathered surfaces show striations parallel to, and caused by the cleavage. Where these striations cross the layers of calcite the layer is given a prismatic appearance that could be confused with the prismatic structure of a Maitaia shell. The general appearance of the fragments is such as would be expected if the distortion of the large brachiopods illustrated by Wilckens (1927, plate IX) from Mt. St. Mary were to be carried several stages further. Although unidentifiable, these fossils are characteristic and may have value for correlation in the sparsely fossiliferous rocks of the Alps. One of us (H. W. W.) has found similar fossil traces in rocks of about the same rank in screes at the road near the summit of Arthurs Pass, twenty-two miles south-west.

Thickness. As the structure is uncertain, the total thickness of the beds can only be roughly estimated. The belt is about two miles wide. Graded bedding determined at many places in the Trent Gorge showed that about two-thirds of the outcrops have top to the east. Therefore, as the beds are all near vertical, their total thickness is probably not less than 3,000 feet, one third of the width of the belt.