Petrology of the Silurian Sediments. 43 



acetylene tetrabroniide S.G. 2.938, following the general method 

 -described by T. Crook, A.R.C.Sc. (Dublin), F.G.S., in Hatch's 

 ■" Petrology of the Sedimentary Rocks." 



The heavy portion was then examined under the microscope in 

 media of different refractive indices. The following minerals were 

 thus recognised : — Tourmaline, zircon, rutile, magnetite, sapphire, 

 topaz, a little biotite and chlorite, some quartz, probably attached 

 to some of the other minerals during flotation, and perhaps kyanite. 



The tourmaline is abundant, and in many cases shows good 

 •crystal boundaries. It contains many microscopic inclusions of 

 •other minerals, and of gas bubbles. It is generally yellow brown 

 to dark brown, but some fragments polarise from a blue green to a 

 pure green. 



Zircon comes next in abundance. The crystals show almost 

 perfect crystal faces and are in many cases zoned. Faces shown 

 include prisms and pyramids. 



Rutile occurs as dark brown jirisms generally, with pinacoidal 

 terminations. 



Magnetite was also noticed in the demagnetised ( f) product. It 

 shows characteristic rectangular outlines. It was probably too small in 

 this case to separate itself efficiently from the sand under magnetic 

 influence. 



Sapphire occurs as deep blue, slightly pleochroic, irregular, 

 nngular grains, showing low polarisation colours, and is fairly 

 plentiful. 



Topaz occurs as rounded and irregular grains, and in many cases 

 lias many inclusions, some dark, which are probably iron ores. 



A little biotite altering to chlorite was found. In one case the 

 •chlorite showed a fine spherulitic structure. 



A doubtful crystal of kyanite was recognised, but as only one grain 

 Avas found it is not considered wise to positively assert its presence. 



3. — Description of Rock Sections. 



^ote. — ^The rock sections are included in the collection of the Geologi- 

 •cal Department of the University. 



(a). Cohurg Specimens. — Rock No. 1 consists of a hard breccia 

 •composed of angular sandstone fragments set in a matrix of finer 

 material largely replaced by iron oxides. 



Microscopically the rock fragments show quartz, muscovite, and 

 tourmaline set in a groundmass of quartz and secondary mica. The 

 rock is clearly a fault breccia. It is to l>e found about 100 yards 



