25 
violently contorted, reminding one very forcibly of the contorted 
jaspilites as seen in the goldfields to the south, an excellent ex- 
am])le of which is to be seen at Marda (Fig. 27). near Mount 
Jackson in the Vilgarn Goldfield. 
Fifi. 27- 
Contoned jaspilite, Marda, Mou'U Jackson, Yil^arn Goldfield. 
A remarkable, and very important feature in the plateau 
region occupied by the Nullaginc Formation is the abundance of 
dolerite intrusions, which take the form either of nearly horizontal 
sills or steeply inclined dykes. 
These dolerites have a remarkably uniform composition, and 
wherever they have been examined th rocks exhibit little or no 
trace of recrystallisation or other signs of metamorphism. Occa- 
sionally a glassy selvage due to rapid cooling may be noticed occur- 
ing at the contact between the dykes and the rocks it traverses. 
The dolerites seem to be in practically the same condition in which 
they originally congealed, and no great terrestrial disturbance 
seems, when the Nullaginc Formation is viewed broadly to have 
effected the region since the time of their injection. The dykes are 
all readily distinguished by their dark-greenish colour, a rusty and 
in places exfoliating weathering. Some extend across country in 
more or less straight lines for many miles, and give rise to fairly 
conspicuous features standing out boldly on the back of the ridge. 
