-10 ON" T1IK QSOLOGV AND PKTROG&APH? OK 15ATHURST, N.S.W., 



The proportion in which the minerals occur, as revealed by the 

 microscope, may be expressed as follows, felspar being the com- 

 monest : — 1. Orthoclase ; -. Silica ; 3, Triclinic felspars ; 4. Bio- 

 tite ; o. Hornblende ; 6, Magnetite ; 7. White mica. 



-pars. — With crossed Nicols, the felspars can be readily 



divided into orthoclase and into felspars with distinct triclinic 

 striatums. The orthoclase occurs in sub-crystalline patches, and, 

 in most slides, is the more plentiful of the two. In its general 

 appearance the orthoclase is always cloudy, even in the thinnest 

 sections. The cloudiness and opacity of the orthoclase is a constant 

 character in all the slices 1 have cut. I attribute this peculiar 

 dimness to pores and fractures that no doubt hasten incipient 

 kaolinir.ation. This structure has, no doubt, a great deal to do 

 with the " sickening of the rock" before referred to. Indeed, 

 anyone accustomed to micro-petrographieal work, would, on 

 account of these characters, at once decide that the rock was not 

 of an enduring character. 



lime Felspars. — The banded appearance, so characteristic of 

 the triclinic felspars, is at once noticeable under crossed Nicols in 

 every slice. The amount of plagioclase relative to the orthoclase 

 varies much. The plagioclase is often in excess, and sometimes 

 the two felspars seem equal in quantity. I sent a few slices of 

 this rock to Mr. A. W. Howitt. our leading Australian petrologist, 

 and he decided, from the structure of the crystals and from their 

 obscuration angles, that the felspar was oligoelase. Sections are 

 not uncommon with the tine bands of colour crossing at an angle of 

 90°. This felspar contains inclusions of other minerals that had 

 ervstallized before itself. Maguetite is a common inclusion, as 

 well as corroded crystals and plates of hornblende. 



Ilornblend''. — Every slice will show hornblende more or less 

 plentifully under the microscope. The crystals are generally much 

 corroded, showing that they were formed long before the felspars. 

 A few examples show the exact prismatic hornbleude. Most of 

 the sections, however, are in zones other than the prismatic, and 

 show only one set of cleavage Hues. In thin sections it appears 



