128 



NOTES ON GEOLOGY AND MINERALO(iY. 



BASALTIC COLUMNS IN WEST DEVON. 



By THOMAS OLAEK, (Associate of the Royal Institution of Cornwall). 



This peculiar structure, I believe, has not hitherto been 

 recorded as having been found in England. 



The fragment exhibited was discovered by Mr. Collins, 

 The College, Eedruth, in a railway cutting between Marytavy 

 and Tavistock, on the London and South Western Eailway. He 

 says, the columns are irregular hexagonal prisms, and are about 

 two, or two and a half feet, across ; they are dipping south, at 

 an angle of about 20° from the perpendicular ; and are found 

 near the junction of the Devonian and carboniferous strata, and 

 with other igneous formations, and intercolated with schistose 

 rocks of a dark shaly character, through which veins of quartz, 

 iron pyrites, and manganese pass. 



The minerals, of which this fragment of basaltic column is 

 composed, are sanidine, labradorite, calcite after sanidine, mag- 

 netite much changed, hornblende, few needles of apatite, and 

 some serpentineous matter probably after olivine. It is its 

 sanidine felspar which gives it a claim to be placed in the 

 tertiary extrusions, and also the fact of its being found on 

 the lines of the north and south volcanic fissures of that period, 

 the remnants of which are so conspicuous in the Western 

 Islands of Scotland, Isle of Man, N.E. of Ireland, Snowdon, 

 and, now, near Tavistock, West Devon. 



This is another link in the chain of evidence connecting the 

 Lizard basic rocks with the Irish and Scotch Tertiary eruptions, 

 and further confirms my former views on the basic rocks of the 

 Lizard district. 



