52 PROCEEDINGS OP THE ROYAL SOCIETY OF QUEENSLAND. 
within the Bnlolo and the Watut valleys (Plates 1 and 2). It reaches 
to the coast at Salamaua, southeast to the Waria River, and is prob- 
ably more or less continuous with the granite masses of the central ranges 
from Ramu to Mount Hagen. Though usually referred to by the field 
term granite, analyses of numerous thin sections cut from specimens 
collected at various parts of the intrusion, from the centre out to the 
margins, shows it to be a slightly acidic granodiorite, or adamellite. 
Average silica content is just below 70 per cent. Differentiated phases 
of the mass have produced, especially around the margins, such types 
as monzonite, diorite, hornblendite, and even a very little pegmatite. This 
granodioritic mass plays a most important role in the economic geology 
of the Morobe goldfield and has been the direct source of a considerable 
proportion of the gold mineralisation. Its age can only be placed with 
any certainty as pre-Tertiary, for it is overlain by Tertiary sediments 
and earlier volcanics west of the Watut River, and the conglomerates 
which comprise part of the Tertiary series contain boulders of the granite. 
In the Wagi Valley similar granites appear from the scanty evidence 
available to underlie both Eocene and Cretaceous strata. The intrusion 
therefore may be as early as late Palaeozoic but it is considered more 
probable that it took place during Mesozoic times. 
TERTIARY SERIES. 
West of the Markham and the Watut Rivers, and in fact through 
most of the Mandated Territory, the old met amorphic complex and the 
major intrusives are largely masked by extensive series of sediments, very 
often accompanied by volcanics of various types. These sediments are 
widely developed and attain great thicknesses during the Miocene and 
Pliocene, but also include strata of Eocene and Oligocene age, when the 
volcanic fraction is more strongly represented. In the Central High- 
lands of New Guinea, Cretaceous and Jurassic beds also occur. 
In the Morobe goldfield area and to the east of it on the mainland 
these marine Tertiaries are not present. They have either been removed 
by erosion, or, as is more probable, most of this area remained a land 
mass while Tertiary deposition was proceeding elsewhere. The composi- 
tion of the Tertiary conglomerates to the west and south and the general 
distribution of the sediments strongly suggest that their component 
materials were derived from such a land mass to the east. 
EARLY PORPHYRIES. 
Within the Bulolo and the Watut valleys the next rocks in point of age 
lo the main granodiorite intrusions are the early porphyries, which are 
lound mainly in the Wau-Edie Creek area. These intrusions comprise well 
crystallised quartz-biotite porphyries, and appear to have been intruded 
at some depth below the surface. The Lower Edie porphyry, for instance, 
outcrops over an area of 7 to 8 square miles, and intense silicification 
of the invaded Kaindi series has taken place along the margins. In 
