Petrology of the Silurian Sediments. 41 



2.— Composition of the Sediments. 



The sandstones, mudstones and shales are all highly aluminous. 

 The principal allothigenic minerals identified microscopically are 

 quartz, more or less fresh felspars, muscovite, and biotite. The 

 accessories noted are tourmaline, iron oxides (magnetite or 

 ihnenite), zircon, rutile. and perhaps anatase and sphene. Secon- 

 dary or authigenic minerals present are sericite. limonite, leucoxene^ 

 quartz, and possibly pyrite, subsequently altered to limonite. Car- 

 bonates may occur to a small extent but no effervescence with acid 

 Avas observed with the powdered rocks. Chlorite after biotite is 

 present in some sections. 



(a). Essential Minerals. — The detrital quartz ranges to about 0.5 

 mm. as a maximum in the sandstones. In some of the sandstones 

 small well developed quartz veins have been formed by secondarj'- 

 solution and redeposition. 



The quartz is angular in habit. It does not show any crystal 

 l>oundaries. It frequently shows strain polarisation effects as the 

 result of pressure. Inclusions of such minerals as apatite are some- 

 times found. 



The felspar is found in two conditions. The greater amount 

 occurs as turbid grey patches throughout the section of about the 

 same size as the quartz, and in some cases in nearly as large an 

 amount. These patches show, in some cases, remains of lamellar 

 twinning. They consist essentially, as far as can be determined, of 

 fine aggregates of secondary sericite. The original felspars have 

 probably been altered by percolating solutions producing secondary 

 mica. In some cases these sericitic patches have been subsequently 

 stained by limonite impregnations. 



A few unaltered grains of oligoclase occur in most of the sections. 

 The refractive index, twin lamellation, and such extinction as can 

 be observed refer it to oligoclase. These again are of the same 

 order of magnitude as the quartz. 



No felspar has l^een identified in the mudstones and shales, though 

 possibly originally present. 



Mica occurs in three forms : — 1, As long ragged clear crystals of 

 muscovite sometimes nipped in between the neighbouring sand 

 grains. These are clearly detrital and average about one millimetre 

 in length. 2, As secondary sericite after felspar, and also through- 

 out the groundmass constituting most of the clayey matter of the 

 groundmass. This may be partly allothigenic and partly authigenic 

 as is certainly the case with that representing felspar remains. 



