The Sutherland Volcanic Pipes. 



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deposition of these minerals is very probably due to the passage 

 of water charged with them after the explosive action had ceased : 

 an analogous process to the sol- 

 fataric stage of ordinary volca- W 

 noes. 



Forty-six dykes filled with 

 material apparently similar in 

 general character to the agglo- 

 merates and tuffs of the vents 

 have been noticed within a short 

 distance of the central peak ; 

 some of them are shown on the 

 plan. The longest dyke is over fv 

 700 yards in length. The ma- 

 jority have a north-north-easterly 

 trend. Few of the dyke rocks 

 have been examined in detail. 

 A specimen taken from the 

 longest dyke is seen under the 

 microscope to be made up of 

 small but well-preserved felspar 

 laths imbedded in a very fine- **i 



grained matrix of interlocking 

 crystals, the nature of which has 

 not been made out, but they may 

 be felspar. The felspar laths 

 are andesine or albite. A por- 

 phyritic crystal of felspar con- ^ 



tains small flakes of strongly 

 pleochroic biotite, the only ferro- 

 magnesian mineral seen in the 

 specimen. Apatite is present, as 

 well as much calcite in the form 

 of minute crystalline grains scat- 

 tered through the matrix and as \ \\ 

 pseudomorphs which show rec- 

 tangular, hexagonal, or irregular 

 boundaries. No other sign of < 

 the original nature of this mineral J 

 has been observed. In addition 

 to these constituents there is much hydrated ferric oxide. This 

 rock is undoubtedly an igneous one, and up to the present it 

 is the only rock from the Saltpetre Kop vents and dykes that 



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