MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 227 



rock is seen to be a crystalline granular compound of large horn- 

 blendes and a rather small quantity of felspar. In fact, the 

 felspar seems merely a cementing matter. Some of the hornblende 

 crystals measure half-an-inch in length. This slice is cut from 

 a rather fine-grained variety of the rock. In all the slices that I 

 have examined the felspars formed but a small part of the whole. 

 There are no idiomorphic felspars. Either in polished blocks or 

 in microscopic slices this is an exceptional and interesting rock. 



Slices 137 to 141 inclusive. Dyke Rocks, from Kaiser, near 

 Wellington, New South Wales. These are slices of a very 

 interesting series of intrusive rocks that occur in the locality 

 named. Slice 139 may be taken as typical of their general 

 character. Under the microscope we can clearly distinguish a 

 micro-crystalline ground-mass and embedded porphyritic crystals. 

 The composition of the ground-mass is as follows : — 1. Broad 

 stretches ofafelsitic-lookingbase, made up of very minute colourless 

 felspar prisms forming an independent net-work. 2. Microlites 

 that are slightly dichroic. 3. Brightly polarizing grains, probably 

 quartz. 4. Opaque matter, pyrites or magnetite. In this ground- 

 mass are porphyritic crystals of felspar, some striated and many 

 of them very clearly twinned in broad plates. There are also 

 larger felspars showing lines of growth resembling a zoned 

 structure. These felspars are water-clear, and remarkably like 

 quartz in ordinary transmitted light. Under crossed nicols their 

 real character is revealed. Mica is also a porphyritic ingredient 

 in flakes from one-thousandth to one-fiftieth part of an inch in 

 length. These micas are strongly dichroic. The felspars contain 

 transparent needle-shaped crystals that agree with the characters 

 of apatite. 



Slices 149 and 150. Felspar-porphyrite, Mount Harris, Lower 

 Macquarie River, New South Wales. This rock was collected at 

 Mount Harris, an isolated and conical hill rising abruptly from 

 the great Tertiary plains of Central New South Wales. In 

 hand specimens it is of a bluish-grey colour, resembling many 

 of the more recent quartz porphyries ; in fact, corroded crystals 



