I 2 
Fig. II. 
Mt, Margaret, Hamersley Range, West Pilbara Goldfield. 
matter, crystallising as dolcrite. which forced its way either along 
the planes of bedding or across the strata in the form of intrusive 
sheets, sills or dykes. The others were contemporaneous with the 
deposition of the associated sedimentarics and include outpourings 
of lavas, ashes and other volcanic ejectmenta. 
d'hc results of field investigation extending over ten years, in 
the enormous area occuiiied by this formation and its equivalents, 
have shown it to he of al)Sorbing interest to the petrographer. and 
the student of volcanic geology, as it was without any doubt the 
most active centre of eru])tive energy, to be found in Western Aus- 
tralia during this geological epoch. The gold-bearing charactei 
of the basal members of the formation has given additional scienti- 
fic interest to the series, and added considerably to its otherwise 
ecuiiomic importance. 
As our exploratory field work has proceeded, coupled with 
detailed mapping in certain economically important localities, the 
true character and significance of the Nullagine bormation ha:- 
been gradually unfolded. Its systematic study opens up a field 
which bids fair to offer a rich harvest, and when the crop has 
been gathered, it \\\U be found that valuable additions have been 
made to the sum total of our knowledge of the igneous rocks and 
phenomena of Western Australia. 
In the neighhnurhood of Nullagine from which the formation 
takes its name, the violent unconformity separating the series from 
the underlying beds is to be seen. The basal member of the 
Nullagine b’ormation is a massive layer of coarse conglomerate 
