OUTLINE OF THE GEOLOGY OF THE MOROBE GOLDFIELDS. 53 
the interior of the intrusion, it is seen in thin sections to be composed of 
large phenocrysts of quartz, plagioclase and biotite, with abundant horn- 
blende, which is developed more conspicuously in some localities than 
others. Plagioclase is andesine, usually rather basic, and the ground 
mass, consisting of felspar, quartz, biotite, hornblende and magnetite, is 
well crystallised. The phenocrysts are strongly developed, with well 
defined crystal outlines. This porphyry shows a general lithological 
similarity both to the earlier granodiorite and to the later biotite 
porphyries. Towards the margins it becomes finer grained and more 
siliceous, with abundant pyrite, and strong silicification and pyritization 
of the slates, together with the appreciable amount of assimilation that 
has taken place, make it difficult at times to distinguish the exact contact 
of the porphyry with the metamorphics. Porphyry bodies of this type 
outcrop in the Lower Edie Creek and Golden Ridges area, also just west 
of Edie Creek, and at the head of Quombo Creek (Plate 2) and in the 
Waria River Valley. 
LATE PORPHYRIES AND VOLCANICS 
Following them, but from their general characteristics much later, 
are further series of porphyritic intrusions. As most of these are 
mutually independent, their exact relationships are difficult to determine, 
but intrusions occurred at two different periods at least, possibly three, 
all of which must, at present, be considered together. Distinction 
between them is made more difficult in that they have mostly been affected 
by hydrothermal alteration in addition to the intense tropical weather- 
ing, and exposures of fresh rocks are scarce. A wide difference in the 
mode of intrusion from that of the Lower Edie porphyry is apparent, 
and it obviously took place much nearer the surface. They are essenti- 
ally of much the same composition, quartz, biotite, hornblende and 
andesine plagioclase, but are not so well crystallised, the ground mass 
is much finer grained, and their relations to the country rock quite 
different. Little contact metamorphism can be seen, the principal effect 
on the host series being pyritization near the contacts, which are often 
marked by gouge, and nearly always clearly defined except where 
intensely weathered. On these principles it is probable that the porphyry 
mass just below the junction of the Edie and the Merri Creeks is earlier 
than the main intrusion above the junction (Plate 2). Contact breccias, 
somewhat silicified, are developed along its margins and it seems gener- 
ally from its appearance in thin section to have been intruded under 
slightly deeper seated conditions. At about the time of the intrusion 
of these porphyries violent volcanic activity commenced in the Edie 
Creek area and vast quantities of agglomerates were produced. Conse- 
quent upon this upheaval the whole valley was dammed up, and most 
of the agglomerates were actually laid down in water, for many of the 
constituent boulders and pebbles, particularly in the marginal areas of 
the agglomerate, show a degree of rounding and resorting greater than 
could have been acquired by mechanical attrition during volcanic 
ejection. These agglomerates consist not only of andesitic or, more 
