Ordovician Sedmients. 57 



Both detrital and secondary mica occurs, the former as long, 

 ragged, cleaved fragments of muscovite up to 1 mm. in length, 

 .and as rounded and ragged plates. Although for the most part 

 quite clear and colourless, it occasionally alters to a pale green 

 chlorite. The muscovite is often found bent and nipped between 

 the quartz grains, and this is characteristic of every section 

 •examined. Biotite occurs in one or two of the sandstones, but 

 is practically all altered to a greenish and brownish chlorite. 



The secondary mica is generally represented by sericite, occur- 

 ring throughout the ground-mass of all the rocks, and making 

 up practically the whole of the slates. The sericite constitutes 

 most of the original clayey matter of the ground-mass of the 

 sandstones, and at times is the result of alteration of the felspars. 

 Some of the plates of muscovite may possibly be secondary. 



(b) Accessory Minerals. — Tourmaline is the dominant acces- 

 sory, and was detected in every section of sandstone. Both the 

 blue and brown pleochroic varieties are represented in grains 

 up to .2 mm. diameter. Generally it occurs as rounded detrital 

 grains, but occasionally it shows traces of crystal boundaries. 

 Only in one case was tourmaline found to occur in slate, and in 

 that instance it was included in secondary arsenopyrite. 



Zircon occurs in all of the sections, never exceeding more 

 than .25 mm. diameter. It is always clear and colourless, and 

 generally slightly rounded, though still showing crystal boun- 

 daries. It is not so abundant in the slates as in the sandstones. 



Rutile occurs in a number of the sections, but rarely exceeds 

 more than .1 mm. diameter. Generally the 'grains are some- 

 what rounded, brown and violet pleochroic tints being common. 



Apatite is a rather constant accessory in many of the sections 

 in grains up to .3 mm. maximum. Although sometimes 

 rounded, it always shows traces of crystal boundaries. 



The determination of sphene in some of the sections is doubt- 

 ful, owing to the difficulty of distinguishing it from zircon in 

 small grains. But one or two boat-shaped crystals, with oblique 

 extinction appear rather definite. 



Ilmenite is quite a common accessory in all the rocks, occur- 

 ring as irregular grains generally altering to leucoxene. Mag- 

 netite also occurs in irregular grains, rarely in minute octahedra. 



Carbonaceous material occurs especially in the slates, and is 

 probably the result of the decay of some form of life in the 

 sediments during their deposition. 



