96 Proceed ill (J X of flie Royal Society of Victoria. 



The porphyritic felspar appears to be normal sanidine ; it 

 contains numerous inclusions, the most common being needles of 

 apatite. Brown hornblende also occurs in phenocrysts, and 

 there aie a few crystals of apatite. 



BrocKs Momiinent. — I have not visited this locality, but from 

 the Quarter-sheet map it would appear that the igneous rock at 

 this place is intrusive through the Silurian rocks. It is the most 

 easterly point of the Macedon district at which I have found 

 trachyte. In appearance it is a blue-gray compact rock, studded 

 with porphyritic crystals of felspar. Microscopically it bears a 

 strong resemblance to the rock described from Dryden's Mount, 

 the chief point of difference being the presence of augite in 

 addition to hornblende — which latter mineral is much altered — 

 and the relative scarcity of apatite. If the one rock is a trachyte 

 so also is the other. 



On the whole it will be seen that trachyte, where it occurs 

 in the Macedon area, is fairly constant both in structure and 

 composition. Glass and free quartz, if present at all, are there 

 in only very minute quantities. Of the geological age of the 

 igneous rocks above described very little is known. They are 

 almost certainly post-Silurian, and from their apparent absence 

 from the boulder-clay so largely developed further north, it seems 

 probable that they were erupted subsequent to the Permo- 

 carboniferous ijlacial ajje. 



Coleraitie District. — Under this title are comprised various 

 localities near Coleraine, for the most part identical with those 

 classed by Mr. Deunant in the map previously referred to. I do 

 not propose to discu.ss in this present paper their complicated 

 field relations, but merely to indicate a few of the salient features 

 of the trachytes of this district. 



A few miles north of Coleraine, and lying just west of the 

 Koonong Wootong Creek, rise two small conical hills, composed of 

 olivine basalt, locally known as Adam and Eve. Through the 

 former of these, on its north side, runs a dyke of light coloured 

 porphyritic rock from west to east. It is from this dyke that I 

 obtained my tii-st specimen of undoubted trachyte in this district. 

 The dyke may be traced west to a low hill where it has been 

 extensively quarried. In the thin slice the phenocrysts of felspar 

 are seen to be mnnoclinic and of the same type as the felspar in 



