BanTRUM.— Geology of .Riverhead-Kaukapakapa District. 145 
sandstones interbedded with frequent thin layers of mudstone. Carbonized 
T 
small-scale folds and by faults which generally have unimportant throw. 
It is difficult amid this complexity to determine the major structure, but 
observations tend to show that the general strike is approximately N. 55° E., 
and that the beds rise successively in the series as followed north-west, at 
all events as far as Riverhead. At this latter place beds of conglomerate 
are intercalated in the sandstones and constitute a conspicuous feature of 
the geology for at least twelve miles northward.* Since his first description 
of their occurrence and petrographic interest, the writer has found them in 
numerous localities, and has recognized several fresh rock-varieties repre 
sented amongst the pebbles (Bartrum, 1920). His first impression was that 
they represented a definite horizon—at a higher level in the series, however, 
than the basal conglomerates near Papakura, and at Motu Tapu, Kawau 
Island, Cape Rodney, and other places where the Waitemata beds rest hard 
upon the mid-Mesozoic basement exposed in those localities. This view is 
almost certainly incorrect, for there are several bands of conglomerate 
exposed near Red Hill and elsewhere, which are separated by variable 
thicknesses of sandstone. Acute disturbance is the keynote of the structure, 
and there is the additional handicap of infrequent outcrops, so that accurate 
identification of horizon is difficult, if not impossible. 
etrographically the conglomerates are characterized by an abundance 
of dioritic pebbles along with various greywackes and argillites, andesites 
andotherrocks. Their texture varies considerably. Not infrequently there 
is a gradual passage from sandstone through grit to fine conglomerate in 
which the pebbles average about lin. in diameter. Generally, however, 
there are numbers of coarse boulders, 3 in. or 4 in. in diameter, along wit 
finer matrix, whilst exceptionally there are incorporated rock-masses 7 ft. 
and more in diameter. The dioritic boulders seldom exceed 1 ft. in diameter, 
whilst the especially large ones are invariably andesitic. In the majority of 
the exposures the freshness of all types of rock incorporated is very notice- 
able, but this statement is not applicable to those outcrops at the higher 
levels where conditions have favoured deep weathering. 
Isolated large boulders of impure jasper, veined freely by small comby 
and drusy veins of quartz, are to be found here and there in areas o 
Waitemata beds. One noted in an easterly headwaters branch of the 
creek draining the north-east slope of Red Hill is unusually large and 
measures at least 25 ft. in diameter. None of these masses was discovered 
in situ, and the only explanation that the writer can offer of their occur- 
rence is that they are local silicifications of the Waitemata beds. It is 
possible that siliceous springs furnished the silica required. 
The distribution of the Waitemata beds is shown on the accompanying 
map, and need not be detailed in full. Their location has often been a 
matter involving an element of speculation because of the scarcity of out- 
crops. This is particularly the case for an area shown extending west from 
Lloyd’s Hill, in the north-east of the map. Waitemata sandstones are 
indicated by soil, topography, and occasional outerop; but much of the 
central portion of this area is elothed in dense forest, and was not 
examined. 
* Cox (1881, p. 27) notes at Riverhead the discovery of ''several specimens of 
volcanic rocks from boulders which appear to be included in the sandy marls." 
