29 
On the northern face of the plateau in the watershed of the 
Maitland River in the West Pilbara Goldfield, may be seen a 
splendid example of one of these dolerite dykes, cutting the hori- 
zontal strata transversely, but unfortunately no opportunity of 
photographing it could be had. 
These dolerite dykes have been noticed in mine workings, 
traversing the auriferous deposits. An excellent instance of which 
may be seen in the workings in the Oroya-Black Range Mine at 
Sandstane. (Fig. 33). The dykes may also be seen at other locali- 
ties outside the limits occupied by the Nullagine Beds, as at the 
Croydon Road Crossing on the Sherlock River. (Fig. 34), 
riff. 34. 
Gabbro Byke, near Croydon Road Crossing, Slierlock River, 
West Pilbara Goldfield. 
In addition to these dykes and sills, there are a large number 
of quartz reefs, containing more or less gold and copper traversing 
the Nullagine Formation, only however, in those special localities 
which have undergone more or less earth movement. 
From such brief descriptions as it has been possible for me to 
give you during the short time at my disposal, it appears that during 
Nullagine Time, there must have been a huge reservoir of molten 
matter lying beneath the surface to the north of latitude 26, and 
merely awaiting a suitable opportunity of rising to the surface. 
There is as yet little definite evidence as to the nature and 
composition of the parent magma, from which these igneous rocks 
were derived, nor any adequate explanation as to why the rocks 
were acidic in some localities and basic in others. 
