1919.] 
Departmental Eeports. 
393 
Mokau Subdivision. 
By J. Henderson, New Zealand Geological Survey. 
The survey of the Mokau Subdivision was continued from last season, and 
field-work was carried on from the 1st January to the 10th May. The 
examination of the survey districts of Awakino, Awakino North, Awakino 
East, Tainui, and Mokau was completed, and these areas, together with 
a small portion of Totoro Survey District, make the Mokau Subdivision. 
Rocks of Mesozoic age, ranging from the Trias to Upper Cretaceous, 
cover a considerable area. They form the Herangi Range, and in the 
north-eastern part of the subdivision, along the valleys of the larger streams, 
appear from beneath the' thin layer of Tertiary rocks. In the Herangi 
Range the rocks in general dip eastward, but in the eastern part of the 
subdivision the dip is westward. Thus the general arrangement of the 
beds is synclinal. 
Tertiary strata overlie the Mesozoic rocks with great unconformity. 
A limestone, undoubtedly the same as the Te Kuiti limestone, covers a 
large area in the north-eastern portion of the subdivision, and extends 
for some miles southward along the valley of the Awakino River. For the 
most part it is separated from the Mesozoic rocks by a varying thick¬ 
ness of conglomerate, grit, and sandstone, but in some localities beds of 
carbonaceous shale with thin coal-seams occur. 
Overlying the limestone are layers of claystone which attain a thickness 
of- 500 ft. or 600 ft. These beds occur in the eastern portion of the sub¬ 
division, and appear along the Awakino Valley resting on the limestone 
above described. In this latter locality are two thin bands of limestone, 
one at the top of the claystone, the other intercalated 400 ft. lower. 
Conformably on the claystone and on the upper limestone band where 
this is present rest massive layers of sandstone which constitute the coal- 
measures of the Mokau River. These cover or underlie without much 
cover a large area in the central and south-eastern portion of the sub¬ 
division. Through them the Mokau has cut a picturesque gorge. Numerous 
bands of carbonaceous shale and several coal-seams occur. These are over- 
lain conformably by well-bedded blue- and green-coloured sandstones which 
contain argillaceous and calcareous material. These attain a maximum 
thickness of about 1,000 ft. 
Elevation of the sea-bottom now took place, followed, after no great 
interval of erosion, by depression. At the same time andesitic ash and 
lapilli were ejected from a neighbouring volcanic vent or series of vents, 
and beds of andesitic tuff were laid down. In many localities the basal 
layer of this series is a calcareous conglomerate, and this passes in places 
into limestone containing but a small amount of volcanic material (Mokau, 
Mohakatino, and Tima Point limestones). Only the lower part of this 
seiies is richly tufaceous, its upper portion consisting of sandstones and 
claystones with their various gradations. Further, the coarsest and thickest 
tuff is to be found near the coast, and near the south-eastern corner of the 
subdivision hornblende and feldspar crystals and fragments scattered rather 
sparsely through argillaceous sandstone indicate the activity of distant 
volcanoes. The beds of this series are well exposed along the coast, and 
also cover a considerable area in the southern portion of the subdivision. 
That an erosion interval separates the beds last described from those 
overlying is proved by the general irregularity of the surface between them 
and by the existence of faults that have affected the lower series but not 
the upper. The rocks of the latter consist, near their base, of massive sand¬ 
stone layers. A sprinkling of hornblende and feldspar fragments shows 
