part 4] 



PLIOCENE DEPOSITS OF CORNWALL. 



365 



(i) Frequency of Occurrence (deduced from the average 

 mineralogical composition of each deposit). 



Abundant. Magnetite, tourmaline, leucoxene, andalusite. 



Common. Ilmenite, zircon, topaz, muscovite, kyanite. 



Scarce. Epidote, rutile, cassiterite, staurolite, anatase. 



Rare. Garnet, chlorite, biotite, brookite. 



Doubtful. Cor andum , xenotime . 



(ii) Persistence of Species: (Distribution Diagram). 



Mineral. 



St Agnes. 



St. Erth. 



St. Keverne. i 



Magnetite 



Ilmenite 



Garnet 



Tourmaline 



Staurolite 



Epidote 



Chlorite 



Biotite 



Leucoxene 



Zircon 





i 



1 











' 

















' i 



— = 



1 



Kyanite 



Anatase 



Rutile 









— — 







Brookite 







Topaz 







Andalusite 







i 



Muscovite 









Corundum 



? 







Cassiterite 





Xenotime 



? 





1 







(iii) Constant Mineralogical Features noted throughout. 



Magnetite. 



Ilmenite. 



Tourmaline. 



■Staurolite. 



Epidote. 



Z i r c o n. 



Kyanite. 



Anatase. 

 Rutile. 



Rounded octahedra or grains. Not ragged. 



Commonly irregular and ragged. 



Prismatic and rounded grains both present. Deep-brown 



basal sections with interference-figures very common. 



Numerous and parti- coloured grains occur in all cases. 

 Well-worn grains with similar physical and optical pro- 

 perties noted throughout. 

 Always sharply angular, greenish-yellow in colour, many 



grains exhibiting partial interference-figures. Weak 



but characteristic pleochroism. 

 Extremely well-rounded prismatic forms characteristic in 



all cases. A few of the ' stumi3y ' prismatic varieties 



always present. 

 Precisely the same type present in both the St. Agnes and 



the St. Erth deposits. The rounded, elongated prisms, 



showing traces of a well-marked (010) cleavage, are 



always present. 

 Commonly in indigo-blue tabular crystals, seldom abraded. 

 Well-rounded ' foxy '-red or reddish-brown, feebly-pleo- 



chroic grains are the prevalent type, with a subordinate 



amount of yellow translucent fragments. 



2c2 



