Notes on the Area of Intrusive Rochs at Dargo. 139 



is partly a very dark-brown basis and partly of exceedingly 

 numerous microliths, being minute, stout, and often twisted 

 fibres or grain s^ the result of devitrification. These are 

 collected together much more in some places than in others. 

 In this ground-mass are : — 



{a.) Magnetite, in rounded crystals and grains. 



(6.) Colourless prisms and crystalline grains of augite. 

 These are not only porphyritic, but also descend in size to 

 almost microscopic dimensions in the ground-mass. In places 

 the augite is in clusters of angular grains, which seem to be 

 the crushed remains of crystals. These clusters of grains 

 are enveloped in the mica next to be mentioned. 



(c.) Dark brown red mica, in which the absorption is 

 strong, the rays being orauge-red, and almost colourless. 



(d.) Serpentinous pseudomorphs, after olivine. In some 

 of them the form of the original mineral is still preserved, 

 together with the meshed appearance produced by the 

 fractures which characterise it. 



(e.) There are also a few crystals of felspar, which are not 

 striated, but which have not the form, and do not obscure 

 after the manner of orthoclase. 



This rock consists essentially of a ground-mass, containing 

 some basis, and having porphyritic crystals of augite and mica 

 in about equal amounts; the olivine about half the amount 

 of either, and the felspar quite subordinate. 



The peculiar features of this rock, and the occurrence of 

 the colourless augite, together with the olivine pseudo- 

 morphs, lead me to think that the rock is pretertiary in age, 

 and, according to the classification I follow in these notes, a 

 very basic variety of Diabase (Olivine- diabase). 



Quartz Diorites. — So far as I have examined them, the 

 massive intrusive rocks of Dargo belong to the diorite 

 group. Most commonly they are light in colour, but in places 

 the mica or the amphibole, or both, increase so much that the 

 rock necessarily has a darker shade of colour than in those 

 examples, in which the felspar and the quartz are more 

 abundant. In almost all cases the rock is a quartz diorite, 

 but I have met with places where the cjuartz almost, if not 

 quite, disappears. Such rocks are found in parts of Orr's 

 Creek, and a sample, to which I shall refer later on, was 

 collected near where the sketch section on Plate III. crosses it. 



As in other parts of Gippsland, these diorite rocks weather 

 much more rapidly than the surrounding sediments, and, as 

 a consequence, the Dargo area forms an extensive basin of 



