BY W. N. BKNSON. 541 



sQ penetrated by the grains of other minerals, particularly biotite, 

 that it cannot be supposed to be a waterworn surface. 



The fourth group is that of the Felspathic Slates. This is 

 perhaps the most interesting of all the groups. They form a 

 large portion of the southern part of the Parish of Lowry, 

 stretching from its \vestern boundary to near the Caloola quarries; 

 they also occur in a rather unusual form at a point where 

 •George's Plains Creek passes from the slates to the granites. 

 Macroscopically, the Felspathic Slates are hard, compact, dark- 

 coloured rocks of rather coarse grain {i.e., the individual grains 

 can be separated by the naked eye), and with poor or bad cleavage, 

 though the parallel arrangement of particles is obvious. Micro- 

 scopically, they are not unlike the augen slates, showing a granular 

 ground-mass and some comparatively large grains of quartz, with 

 indications of a rock-flowage structure. Biotite is in plenty in 

 green-brown flakes, and muscovite is in smaller flakes. The 

 usual accessory minerals, rutile, zircon, and sometimes magnetite 

 occur; also a good deal of andalusite. Orthoclase and oligoclase 

 are developed in large idiomorphic and abundant crystals, twinned 

 in- their usual fashion, and very full of inclusions frequently 

 arranged in bands parallel to the direction of schistosity, and 

 irrespective of the orientation of the felspar crystal. 



The unusual variety from the creek is characterised by its very 

 typical rock-flowage structure, and the abundance of large oval 

 masses of orthoclase containing oval inclusions of quartz arranged 

 parallel to the schistosity. Muscovite and magnetite occur. 

 Albite is also present, but subordinate to the orthoclase. Further, 

 long ropy bands of very fine, almost ultra-microscopic, colourless 

 needles go through the rock in the direction of schistosity, but 

 bending in and out among the larger grains. These needles 

 would be called sillimanite were it not that they are not fractured, 

 and are sometimes even bent. They may be tremolite. 



As to the origin of these slates, the structure is so unlike that 

 of an ordinary slate that we can hardly believe them to be merely 

 altered sediment. It seems more probable that the}^ were 

 originally tuffaceous sediments of a rather acid type; subsequent 



