362 



MR. H. B. MILNER ON THE 



[vol. Ixxviii, 



Brookite. — Only one grain of this mineral wsis identified in the samples 

 examined, and this occurred in the white sand associated with rutile, as an 

 irregular, well-rounded, yellow crystal flattened parallel to (100) and striated 

 parallel to the vertical axis. The grain is feebly pleochroic, possesses a very 

 high refractive index and birefringence, and exhibits a well-defined biaxial 

 positive interference-figure. 



T o p a z. — This mineral is commoner in the upper horizons than in the lower, 

 and in some cases occurs in excess of the andalusite. The grains are usually 

 somewhat irregular, but are clear, colourless, void of inclusions, and average 

 from 0'2 to 0'3 mm. in diameter. 



Andalusite. — The chief feature of this species, as occurring here, is the 

 intense blood-red pleochroism exhibited by nearly all the crystals, which are 

 in this, and in all other respects, similar to those of St. Agnes. 



(c) St. Keverne. 



Microscopical examination of the Crousa-Comnion deposits gave 

 the following results : — 



Light Material : 

 S.G. <2-82. 



Horizon 



1 

 A. 



B. 



C. 



D. 





Minerals. 

 Quartz 



Essential constituent throughout. 

 5 5 5 5 

 3 2 3 2 



Essential autliigenous material. 



Muscovite 



Glauconite 



Ferruginous cement 





.. 00 



Magnetite 



7 

 8 



8 

 1 

 4 



1 

 2 



7 

 8 



8 

 1 

 4 

 1 

 1 



8 



8 

 1 

 4 

 1 

 1 



7 

 8 



8 

 1 

 4 



1 

 1 



q; c5 



Ilmenite 



Heavy Residue : Heavy Residi 

 Non-Magnetic. S.G.>2-82. Magnetic. S.G.> 



Garnet , 



Tourmaline 



Staurolite 



Epidote 



Chlorite 



Biotite 





Leucoxene 



7 

 4 



5 

 4 



4 

 8 

 5 



4 



7 

 4 



5 



4 



6 

 9 

 5 



4 



8 

 5 



8 

 5 

 2 

 5 



9 



7 



1 



7 

 4 



6 

 4 



I 



9 

 5 



4 



Zircon 



Kyanite 



Anatase 



Rutile 



Brookite 



Topaz 



Andalusite 



Muscovite 



Corundum 



Ca ssiterite 



Xenotime 





A. ' Gravel ' from Crousa Common, St. Keverne. (Average of several samples.) 



B. ' Gravel ' from a quarry bj' the main road, 9th milestone from Helston, 



St. Keverne. 



C. ' Gravel ' from the base of the deposits, same locality as B. 

 1). Average composition of the St. Keverne deposits. 



[Conventional signs as in the previous tables.] 



