56 J. A. Dunn : . 



2. Composition of the Sediments. 



Secondary silica has, in many cases, altered the character of 

 the original sediments, but it is quite apparent that as a whole 

 both the sandstones and slates were highly aluminous. The 

 principal minerals identified microscopically are quartz, felspar, 

 muscovite, and biotite (generally altered to chlorite). The 

 accessories detected are tourmaline, zircon, rutile, ilmenite (often 

 altering to leucoxene), magnetite, apatite and sphene. A small 

 pale-bluish isotropic mineral with very high refractive index 

 was detected in one section of sandstone — this is probably blue 

 spinel. Secondary minerals present are quartz, pyrite, arseno- 

 pyrite, sphalerite, galena, chlorite and a carbonate probably 

 ankerite. Sericite constitutes practically the whole of the 

 ground-mass of the slates, leucoxene often appears secondary 

 to ilmenite, whilst chlorite generally occurs after biotite. In 

 a number of the slates, particularly those found on the " backs," 

 black carbonaceous matter constitutes an integral part of the 

 rock, and generally occurs in thin lamellae. 



(a) Essential Minerals examined in thin sections. — The 

 detrital quartz and felspars range to about .7 mm. as a maxi- 

 mum in the standstones and mica often occurs in long, thin 

 ragged plates up to 1 mm. in length. 



The quartz is rounded to sub-angular in habit generally, but 

 often where secondary it becomes sharply angular. Only in 

 very rare instances does it show crystal boundaries. Strain 

 polarisation is rarely evident except in some of the secondary 

 quartz. The characteristic serial arrangement of inclusions is 

 often noticeable, and apatite, zircon and rutile are occasionally 

 found as inclusions. Thin veins of quartz often traverse both 

 sandstones and slates — these veins are in part the result of re- 

 placement, and in part of growth by force of crystallisation. 



The felspar is in much less quantity than the quartz, and on 

 the whole the individual grains are smaller. Occasionally the 

 felspars are in very turbid grey patches, but generally they are 

 rather fresh and represented by both orthoclase and plagioclase. 

 The plagioclase ranges from andesine to oligoclase as shown by 

 the angle of extinction, and is not so abundant as the orthoclase. 

 The felspars are almost universally rounded in habit, but where 

 they are probably secondary, they become quite angular, as in 

 the case of quartz. The alteration of the felspars is as a rule 

 to sericite, but occasionally it goes to calcite. 



