358 
PACIFIC SCIENCE, Vol. IV, October, 1950 
is barely covered by an extreme high spring 
tide. Crab Island (so-called because of the 
abundance of the large shore crab, Lepto- 
grapsus variegatus ) is a small island of rock 
which, although slightly more elevated than 
the Main Reef, is covered by even the lowest 
high tides. At no point is the Main Reef 
higher than M.S.L. (6.22 feet above Auck- 
land Harbour Board Datum). 
The rock is a heterogeneous, fine, volcanic 
agglomerate, locally known as Parnell Grit 
(Bartrum and Turner, 1928). This is con- 
formably interbedded with Waitemata Sand- 
stone, which forms the major part of the 
Auckland Isthmus. There are a large number 
of irregular joints in the rock, along which 
deposition of brittle iron compounds has 
taken place. 
The Auckland district has a maritime cli- 
mate, with a relatively small daily and an- 
nual variation in temperature and precipita- 
tion (Beveridge and Chapman, 1950). Pre- 
vailing westerly winds are off-shore at Nar- 
row Neck, and hence wave action is usually 
slight. Sea temperature closely follows that 
of the surrounding land. 
recorded over two 24-hour periods in the month 
of February, 1947, by the self-registering tide 
gauge at Queen’s Wharf, Auckland. 
There are no fresh-water outlets which 
would have a significant effect on the salinity 
of seawater in the locality. Isolated determi- 
nations were made by Hounsell (1935) at 
North Head, 2 miles south of Narrow Neck, 
and at Rangitoto Beacon to the east, where 
the values were 35.00 and 35.10, respectively. 
Both samples were collected in the morning. 
The pH of seawater at Narrow Neck lies 
between 8.0 and 8.1, the value remaining 
constant throughout the year (Ambler and 
Chapman, in press). 
Tides in the Hauraki Gulf are semi- 
diurnal, with an extreme spring range of 
about 12 feet and an extreme neap range of 
about 5 feet. Day and night tides differ 
slightly in amplitude, the night tides being 
greater in summer and smaller in winter. 
Figure 2 shows typical extreme neap and 
spring tides recorded for the locality by the 
Auckland Harbour Board. It was found from 
readings on a tide pole at Narrow Neck that 
there is no significant disparity in time and 
rate of tidal rise and fall in comparison with 
the recordings on the self-registering tide 
gauge at Queen’s Wharf. 
Tide levels were averaged from Auckland 
Harbour Board marigrams for the years 
1945, 1946, and 1947 in order to avoid using 
data for 1 year only. The method of calculat- 
ing the levels was that used by Beveridge ( in 
Beveridge and Chapman, 1950). There is 
no significant difference between the figures 
for 3 years presented in Table 1 and those of 
Beveridge and Chapman for the year 1945. 
Extreme figures in Table 1 are means of 
monthly extremes. All heights are expressed 
in feet above Auckland Harbour Board 
Datum (0.00 feet, or Extraordinary Low 
Water Spring Tide). 
TABLE 1 
Tidal Data for Auckland Harbour 
HEIGHT IN FEET 
TIDE LEVEL ABOVE A.H.B.D. 
Extreme High Water Springs 
(E.H.W.S.) 11.95 
